<PAGE>

                                                                    EXHIBIT 4.11

                                                                  EXECUTION COPY

                          REGISTRATION RIGHTS AGREEMENT

                  THIS REGISTRATION RIGHTS AGREEMENT is made and entered into as
of September 8, 2003 by and between OSI Pharmaceuticals, Inc., a Delaware
corporation ("the Company"), and Merrill Lynch & Co., Merrill Lynch, Pierce,
Fenner & Smith Incorporated and Morgan Stanley & Co. Incorporated (collectively,
the Initial Purchasers, and each individually, an "Initial Purchaser") in
connection with the Purchase Agreement, dated September 3, 2003 (the "Purchase
Agreement"), between the Company and the Initial Purchasers. In order to induce
the Initial Purchasers to enter into the Purchase Agreement, the Company has
agreed to provide the registration rights set forth in this Agreement. The
execution of this Agreement is a condition to the closing under the Purchase
Agreement.

                  The Company agrees with the Initial Purchasers, for its
benefit as Initial Purchasers and for the benefit of the beneficial owners
(including the Initial Purchasers) from time to time of the Registrable
Securities (as defined herein) (each of the foregoing a "Holder" and together
the "Holders"), as follows:

                  Section 1. Definitions. Capitalized terms used herein without
definition shall have their respective meanings set forth in the Purchase
Agreement. As used in this Agreement, the following terms shall have the
following meanings:

                  "Affiliate" means, with respect to any specified person, an
"affiliate," as defined in Rule 144, of such person.

                  "Amendment Effectiveness Deadline Date" has the meaning
specified in Section 2(d) hereof.

                  "Business Day" means each Monday, Tuesday, Wednesday, Thursday
and Friday that is not a day on which banking institutions in The City of New
York are authorized or obligated by law or executive order to close.

                  "Common Stock" means the shares of Common Stock, $0.01 par
value, of the Company and any other shares of common stock as may constitute
"Common Stock" for purposes of the Indenture, including the Underlying Common
Stock.

                  "Conversion Price" has the meaning assigned to that term in
the Indenture.

                  "Damages Accrual Period" has the meaning specified in Section
2(e) hereof.

                  "Damages Payment Date" means each Interest Payment Date.

                  "Deferral Notice" has the meaning specified in Section 3(i)
hereof.

                  "Deferral Period" has the meaning specified in Section 3(i)
hereof.

<PAGE>

                  "Effectiveness Deadline Date" has the meaning specified in
Section 2(a) hereof.

                  "Effectiveness Period" means the period of two years from the
Issue Date or such shorter period that will terminate upon the earliest of the
following: (A) when all Registrable Securities covered by one or more Shelf
Registration Statements have been sold pursuant to such Shelf Registration
Statement(s) and (B) when, in the written opinion of counsel to the Company, all
outstanding Registrable Securities held by persons which are not Affiliates of
the Company may be resold without registration under the Securities Act pursuant
to Rule 144(k) under the Securities Act or any successor provision thereto.

                  "Event" has the meaning specified in Section 2(e) hereof.

                  "Event Termination Date" has the meaning specified in Section
2(e) hereof.

                  "Event Date" has the meaning specified in Section 2(e) hereof.

                  "Exchange Act" means the Securities Exchange Act of 1934, as
amended, and the rules and regulations of the SEC promulgated thereunder.

                  "Filing Deadline Date" has the meaning specified in Section
2(a) hereof.

                  "Holder" has the meaning specified in the second paragraph of
this Agreement.

                  "Indenture" means the Indenture dated as of the date hereof
between the Company and The Bank of New York, as trustee, pursuant to which the
Securities are being issued.

                  "Initial Purchasers" means Merrill Lynch & Co., Merrill Lynch,
Pierce, Fenner & Smith Incorporated and Morgan Stanley & Co. Incorporated.

                  "Initial Shelf Registration Statement" has the meaning
specified in Section 2(a) hereof. All references to "Initial Shelf Registration
Statement" in this Agreement shall include the Prospectus forming a part of such
Initial Shelf Registration Statement, and any amendments or supplements to such
Initial Shelf Registration Statement, including post-effective amendments, all
exhibits, and all materials incorporated by reference or explicitly deemed to be
incorporated by reference in such Initial Shelf Registration Statement.

                  "Interest Payment Date" shall have the meaning as set forth in
the Indenture.

                  "Issue Date" means September 8, 2003.

                  "Liquidated Damages Amount" has the meaning specified in
Section 2(e) hereof.

                  "Material Event" has the meaning specified in Section 3(i)
hereof.

                  "Notice and Questionnaire" means a written notice delivered to
the Company containing substantially the information called for by the Selling
Securityholder Notice and

                                       2
<PAGE>

Questionnaire attached as Annex A to the Offering Memorandum of the Company
dated September 3, 2003 relating to the Securities.

                  "Notice Holder" means, on any date, any Holder that has
delivered a Notice and Questionnaire to the Company on or prior to such date.

                  "Principal Amount" means, with the respect to the Securities,
the principal amount due on the maturity date as shown on such Securities.

                  "Prospectus" means the prospectus included in any Registration
Statement (including, without limitation, a prospectus that discloses
information previously omitted from a prospectus filed as part of an effective
registration statement in reliance upon Rule 415 promulgated under the
Securities Act), as amended or supplemented by any amendment or prospectus
supplement, including post-effective amendments, and all materials incorporated
by reference or explicitly deemed to be incorporated by reference in such
Prospectus.

                  "Purchase Agreement" has the meaning specified in the first
paragraph of this Agreement.

                  "Record Holder" means, with respect to any Damages Payment
Date relating to any Securities or Underlying Common Stock as to which any
Liquidated Damages Amount has accrued, the registered holder of such Securities
or Underlying Common Stock, as the case may be, at the close of business on the
March 1 and September 1, whether or not a Business Day, immediately preceding
the relevant Interest Payment Date.

                  "Registrable Securities" means the Securities, until such
Securities have been converted or exchanged, and the Underlying Common Stock and
any securities into or for which such securities have been converted or
exchanged, and any security issued with respect thereto upon any stock dividend,
split or similar event until, in the case of any such security, the earliest of
(i) its effective registration under the Securities Act and resale in accordance
with the Registration Statement covering it, (ii) expiration of the holding
period that would be applicable thereto under Rule 144(k) were it not held by an
Affiliate of the Company or (iii) its sale to the public pursuant to Rule 144.

                  "Registration Expenses" has the meaning specified in Section 5
hereof.

                  "Registration Statement" means any registration statement of
the Company that covers any of the Registrable Securities pursuant to the
provisions of this Agreement, including the Prospectus, amendments and
supplements to such registration statement, including post-effective amendments,
all exhibits, and all materials incorporated by reference or explicitly deemed
to be incorporated by reference in such registration statement.

                  "Restricted Securities" has the meaning assigned to that term
in Rule 144.

                  "Rule 144" means Rule 144 under the Securities Act, as such
Rule may be amended from time to time, or any similar rule or regulation
hereafter adopted by the SEC.

                                       3
<PAGE>

                  "Rule 144A" means Rule 144A under the Securities Act, as such
Rule may be amended from time to time, or any similar rule or regulation
hereafter adopted by the SEC.

                  "SEC" means the U.S. Securities and Exchange Commission and
any successor agency.

                  "Securities" means the 3 1/4% Convertible Senior Subordinated
Notes due 2023 of the Company to be purchased pursuant to the Purchase
Agreement.

                  "Securities Act" means the Securities Act of 1933, as amended,
and the rules and regulations of the SEC promulgated thereunder.

                  "Shelf Registration Statement" has the meaning specified in
Section 2(a) hereof, and which shall include the Initial Shelf Registration
Statement and any Subsequent Shelf Registration Statement. All references to
"Shelf Registration Statement" in this Agreement shall include the Prospectus
forming a part of such Shelf Registration Statement, and any amendments or
supplements to such Shelf Registration Statement, including post-effective
amendments, all exhibits, and all materials incorporated by reference or
explicitly deemed to be incorporated by reference in such Shelf Registration
Statement.

                  "Subsequent Shelf Registration Statement" has the meaning
specified in Section 2(b) hereof. All references to "Subsequent Shelf
Registration Statement" in this Agreement shall include the Prospectus forming a
part of such Subsequent Shelf Registration Statement, and any amendments or
supplements to such Subsequent Shelf Registration Statement, including
post-effective amendments, all exhibits, and all materials incorporated by
reference or explicitly deemed to be incorporated by reference in such
Subsequent Shelf Registration Statement.

                  "TIA" means the Trust Indenture Act of 1939, as amended.

                  "Trustee" means The Bank of New York (or any successor
entity), the Trustee under the Indenture.

                  "Underlying Common Stock means the Common Stock into which the
Securities are convertible or issued upon any such conversion.

                  Section 2. Shelf Registration. (a) The Company shall prepare
and file or cause to be prepared and filed with the SEC, as soon as practicable
but in any event by the date (the "Filing Deadline Date") that is ninety (90)
days after the Issue Date, a Registration Statement for an offering to be made
on a delayed or continuous basis pursuant to Rule 415 of the Securities Act (a
"Shelf Registration Statement") registering the resale from time to time by
Holders thereof of the Registrable Securities (the "Initial Shelf Registration
Statement"). The Initial Shelf Registration Statement shall be on Form S-3 or
another appropriate form permitting registration of such Registrable Securities
for resale by such Holders in accordance with the methods of distribution
elected by the Holders and set forth in the Initial Shelf Registration
Statement. The Company shall use reasonable best efforts to cause the Initial
Shelf Registration Statement to be declared effective under the Securities Act
as promptly as is practicable but in any event by the date (the "Effectiveness
Deadline Date") that is one hundred and eighty (180)

                                       4
<PAGE>

days after the Issue Date, and to keep the Initial Shelf Registration Statement
(or any Subsequent Shelf Registration Statement, if applicable) continuously
effective under the Securities Act until the expiration of the Effectiveness
Period; provided, however, that no Holder shall be entitled to have the
Registrable Securities held by it covered by such Shelf Registration Statement
unless such Holder shall have provided a Notice and Questionnaire in accordance
with Section 2(d) and is in compliance with Section 4. None of the Company's
security holders (other than the Holders of Registrable Securities) shall have
the right to include any of the Company's securities in the Shelf Registration
Statement to be filed pursuant to this Section 2(a).

                  (b)      If the Initial Shelf Registration Statement or any
Subsequent Shelf Registration Statement ceases to be effective for any reason at
any time during the Effectiveness Period (other than because all Registrable
Securities registered thereunder shall have been resold pursuant thereto or
shall have otherwise ceased to be Registrable Securities), the Company shall use
all reasonable efforts to obtain the prompt withdrawal of any order suspending
the effectiveness thereof, and in any event shall within thirty (30) days of
such cessation of effectiveness amend the Shelf Registration Statement in a
manner reasonably expected to obtain the withdrawal of the order suspending the
effectiveness thereof, or file an additional Shelf Registration Statement
covering the Registrable Securities (a "Subsequent Shelf Registration
Statement"). If a Subsequent Shelf Registration Statement is filed, the Company
shall use all reasonable efforts to cause the Subsequent Shelf Registration
Statement to become effective as promptly as is practicable after such filing
and to keep such Registration Statement (or any other Subsequent Shelf
Registration Statement) continuously effective until the end of the
Effectiveness Period.

                  (c)      The Company shall supplement and amend any Shelf
Registration Statement filed pursuant to this Agreement if required by the
rules, regulations or instructions applicable to the registration form used by
the Company for such Shelf Registration Statement, if required by the Securities
Act or, to the extent to which the Company does not reasonably object, as
reasonably requested by the Initial Purchasers or by the Trustee on behalf of
the registered Holders, and shall file Subsequent Shelf Registration Statements
in order to enable Notice Holders to effect resale of their Registrable
Securities in accordance with the provisions hereof, if required by SEC rules
and regulations or pursuant to interpretations of the staff of the SEC.

                  (d)      Each Holder of Registrable Securities agrees that if
such Holder wishes to sell Registrable Securities pursuant to a Shelf
Registration Statement and related Prospectus, it will do so only in accordance
with this Section 2(d) and Section 3(i). Each Holder of Registrable Securities
wishing to sell Registrable Securities pursuant to a Shelf Registration
Statement and related Prospectus agrees to deliver a Notice and Questionnaire to
the Company at least three (3) Business Days prior to any intended distribution
of Registrable Securities under the Shelf Registration Statement. From and after
the date the Initial Shelf Registration Statement is declared effective, the
Company shall, as promptly as is practicable after the date a Notice and
Questionnaire is delivered, and in any event within five (5) Business Days after
such date (or within thirty (30) Business Days after such date in connection
with the filing of an additional Shelf Registration Statement pursuant to this
Section, if required by SEC rules and regulations or pursuant to interpretations
of the staff of the SEC), (i) if required by applicable law, file with the SEC a
post-effective amendment to the Shelf Registration Statement or an additional
Shelf Registration Statement or prepare and, if required by applicable law, file
a supplement to the

                                       5
<PAGE>

related Prospectus or a supplement or amendment to any document incorporated
therein by reference or file any other required document so that the Holder
delivering such Notice and Questionnaire is named as a selling security holder
in a Shelf Registration Statement in such a manner as to permit such Holder to
deliver such Prospectus to purchasers of the Registrable Securities in
accordance with applicable law and, if the Company shall file an additional
Shelf Registration Statement or a post-effective amendment to the Shelf
Registration Statement, use all reasonable efforts to cause such additional
Shelf Registration Statement or post-effective amendment to be declared
effective under the Securities Act as promptly as is practicable, but in any
event by the date (the "Amendment Effectiveness Deadline Date") that is thirty
(30) days after the date such additional Shelf Registration Statement or
post-effective amendment is required by this clause to be filed; (ii) provide
such Holder copies of any documents filed pursuant to Section 2(d)(i); and (iii)
notify such Holder as promptly as practicable after the effectiveness under the
Securities Act of any additional Shelf Registration Statement and post-effective
amendment filed pursuant to Section 2(d)(i); provided that if such Notice and
Questionnaire is delivered during a Deferral Period, the Company shall so inform
the Holder delivering such Notice and Questionnaire and shall take the actions
set forth in clauses (i), (ii) and (iii) above upon expiration of the Deferral
Period in accordance with Section 3(i), provided, further, that if under
applicable law the Company has more than one option as to the type or manner of
making any such filing, it will make the required filing or filings in the
manner or of a type that is reasonably expected to result in the earliest
availability of the Prospectus for effecting resales of Registrable Securities.
Notwithstanding anything contained herein to the contrary, the Company shall be
under no obligation to name any Holder that is not a Notice Holder as a selling
security holder in any Registration Statement or related Prospectus; provided,
however, that any Holder that becomes a Notice Holder pursuant to the provisions
of Section 2(d) of this Agreement (whether or not such Holder was a Notice
Holder at the time the Registration Statement was declared effective) shall be
named as a selling security holder in the Registration Statement or related
Prospectus in accordance with the requirements of this Section 2(d).

                  (e)      The parties hereto agree that the Holders of
Registrable Securities will suffer damages, and that it would not be feasible to
ascertain the extent of such damages with precision, if (i) the Initial Shelf
Registration Statement has not been filed on or prior to the Filing Deadline
Date, (ii) the Initial Shelf Registration Statement has not been declared
effective under the Securities Act on or prior to the Effectiveness Deadline
Date, (iii) the Company has failed to perform its obligations set forth in
Section 2(d) hereof within the time period required therein, (iv) the aggregate
duration of Deferral Periods in any period exceeds the number of days permitted
in respect of such period pursuant to Section 3(i) hereof or (v) the number of
Deferral Periods in any period exceeds the number permitted in respect of such
period pursuant to Section 3(i) (each of the events of a type described in any
of the foregoing clauses (i) through (v) are individually referred to herein as
an "Event," and the Filing Deadline Date in the case of clause (i), the
Effectiveness Deadline Date in the case of clause (ii), the date by which the
Company is required to perform its obligations set forth in Section 2(d) in the
case of clause (iii) (including the filing of any post-effective amendment prior
to the Amendment Effectiveness Deadline Date), the date on which the aggregate
duration of Deferral Periods in any period exceeds the number of days permitted
by Section 3(i) hereof in the case of clause (iv), and the date of the
commencement of a Deferral Period that causes the limit on the number of
Deferral Periods in any period under Section 3(i) hereof to be exceeded in the
case of clause (v), being referred to

                                       6
<PAGE>

herein as an "Event Date"). Events shall be deemed to continue until the "Event
Termination Date," which shall be the following dates with respect to the
respective types of Events: the date the Initial Shelf Registration Statement is
filed in the case of an Event of the type described in clause (i), the date the
Initial Shelf Registration Statement is declared effective under the Securities
Act in the case of an Event of the type described in clause (ii), the date the
Company performs its obligations set forth in Section 2(d) in the case of an
Event of the type described in clause (iii) (including, without limitation, the
date the additional Shelf Registration Statement or relevant post-effective
amendment to the Shelf Registration Statement is declared effective under the
Securities Act), termination of the Deferral Period that caused the limit on the
aggregate duration of Deferral Periods in a period set forth in Section 3(i) to
be exceeded in the case of the commencement of an Event of the type described in
clause (iv), and termination of the Deferral Period the commencement of which
caused the number of Deferral Periods in a period permitted by Section 3(i) to
be exceeded in the case of an Event of the type described in clause (v).

                  Accordingly, commencing on (and including) any Event Date and
ending on (but excluding) the next date on which there are no Events that have
occurred and are continuing (a "Damages Accrual Period"), the Company agrees to
pay, as liquidated damages and not as a penalty, an amount (the "Liquidated
Damages Amount"), payable on the Damages Payment Dates to Record Holders,
accruing for each portion of such Damages Accrual Period beginning on and
including a Damages Payment Date (or, in respect of the first time that the
Liquidation Damages Amount is to be paid to Holders on a Damages Payment Date as
a result of the occurrence of any particular Event, from the Event Date) and
ending on but excluding the first to occur of (A) the date of the end of the
Damages Accrual Period or (B) the Next Damages Payment Date, at a rate per annum
equal to one-quarter of one percent (0.25%) for the first 90-day period from the
Event Date, and thereafter at a rate per annum equal to one-half of one percent
(0.5%), of the aggregate Principal Amount of such Securities or, if the Holder
has converted such Securities into Underlying Common Stock and such shares of
Common Stock then constitute Registrable Securities, the Conversion Price of
such Securities in effect at the time of such conversion, in each case,
determined as of the Business Day immediately preceding the next Damages Payment
Date; provided, that any Liquidated Damages Amount accrued with respect to any
Securities or portion thereof called for redemption on a redemption date or
converted into Underlying Common Stock on a conversion date prior to the Damages
Payment Date, shall, in any such event, be paid instead to the Holder who
submitted such Securities or portion thereof for redemption or conversion on the
applicable redemption date or conversion date, as the case may be, on such date
(or promptly following the conversion date, in the case of conversion).
Notwithstanding the foregoing, no Liquidated Damages Amounts shall accrue as to
any Registrable Security from and after the earlier of (x) the date such
security is no longer a Registrable Security and (y) expiration of the
Effectiveness Period. The rate of accrual of the Liquidated Damages Amount per
year shall not exceed the 0.5% notwithstanding the occurrence of multiple
concurrent Events. Following the cure of all Events requiring the payment by the
Company of Liquidated Damages Amounts to the Holders of Registrable Securities
pursuant to this Section, the accrual of Liquidated Damages Amounts will cease
(without in any way limiting the effect of any subsequent Event requiring the
payment of Liquidated Damages Amount by the Company).

                  The Trustee shall be entitled, on behalf of Holders of
Securities or Underlying Common Stock, to seek any available remedy for the
enforcement of this Agreement, including

                                       7
<PAGE>

for the payment of any Liquidated Damages Amount. Nothing shall preclude a
Notice Holder or Holder of Registrable Securities from pursuing or obtaining
specific performance or other equitable relief with respect to this Agreement.

                  All of the Company's obligations set forth in this Section
2(e) that are outstanding with respect to any Registrable Security at the time
such security ceases to be a Registrable Security shall survive until such time
as all such obligations with respect to such security have been satisfied in
full (notwithstanding termination of this Agreement pursuant to Section 8(k)).

                  The parties hereto agree that the liquidated damages provided
for in this Section 2(e) constitute a reasonable estimate of the damages that
may be incurred by Holders of Registrable Securities by reason of the failure of
the Shelf Registration Statement to be filed or declared effective or available
for effecting resales of Registrable Securities in accordance with the
provisions hereof.

                  Section 3. Registration Procedures. In connection with the
registration obligations of the Company under Section 2 hereof, the Company
shall:

                  (a)      Before filing any Registration Statement or
         Prospectus or any amendments or supplements thereto with the SEC,
         furnish to the Initial Purchasers copies of all such documents proposed
         to be filed and use reasonable efforts to reflect in each such document
         when so filed with the SEC such comments as the Initial Purchasers
         reasonably shall propose within three (3) Business Days of the delivery
         of such copies to the Initial Purchasers.

                  (b)      Prepare and file with the SEC such amendments and
         post-effective amendments to each Registration Statement as may be
         necessary to keep such Registration Statement continuously effective
         for the applicable period specified in Section 2(a); cause the related
         Prospectus to be supplemented by any required Prospectus supplement,
         and as so supplemented to be filed pursuant to Rule 424 (or any similar
         provisions then in force) under the Securities Act; and use all
         reasonable efforts to comply with the provisions of the Securities Act
         applicable to it with respect to the disposition of all securities
         covered by such Registration Statement during the Effectiveness Period
         in accordance with the intended methods of disposition by the sellers
         thereof set forth in such Registration Statement as so amended or such
         Prospectus as so supplemented.

                  (c)      As promptly as practicable give notice to the Notice
         Holders and the Initial Purchasers (i) when any Prospectus, Prospectus
         supplement, Registration Statement or post-effective amendment to a
         Registration Statement has been filed with the SEC and, with respect to
         a Registration Statement or any post-effective amendment, when the same
         has been declared effective, (ii) of any request, following the
         effectiveness of the Initial Shelf Registration Statement under the
         Securities Act, by the SEC or any other federal or state governmental
         authority for amendments or supplements to any Registration Statement
         or related Prospectus or for additional information, (iii) of the
         issuance by the SEC or any other federal or state governmental
         authority of any stop order suspending the effectiveness of any
         Registration Statement or the initiation or

                                       8
<PAGE>

         threatening of any proceedings for that purpose, (iv) of the receipt by
         the Company of any notification with respect to the suspension of the
         qualification or exemption from qualification of any of the Registrable
         Securities for sale in any jurisdiction or the initiation or
         threatening of any proceeding for such purpose, (v) of the occurrence
         of (but not the nature of or details concerning) a Material Event
         (provided, however, that no notice by the Company shall be required
         pursuant to this clause (v) in the event that the Company either
         promptly files a Prospectus supplement to update the Prospectus or a
         Form 8-K or other appropriate Exchange Act report that is incorporated
         by reference into the Registration Statement, which, in either case,
         contains the requisite information with respect to such Material Event
         that results in such Registration Statement no longer containing any
         untrue statement of material fact or omitting to state a material fact
         necessary to make the statements contained therein not misleading) and
         (vi) of the determination by the Company that a post-effective
         amendment to a Registration Statement will be filed with the SEC, which
         notice may, at the discretion of the Company (or as required pursuant
         to Section 3(i)), state that it constitutes a Deferral Notice, in which
         event the provisions of Section 3(i) shall apply.

                  (d)      Use all reasonable efforts to obtain the withdrawal
         of any order suspending the effectiveness of a Registration Statement
         or the lifting of any suspension of the qualification (or exemption
         from qualification) of any of the Registrable Securities for sale in
         any jurisdiction in which they have been qualified for sale, in either
         case at the earliest possible moment.

                  (e)      If reasonably requested by the Initial Purchasers or
         any Notice Holder, promptly as reasonably practicable incorporate in a
         Prospectus supplement or post-effective amendment to a Registration
         Statement such information as the Initial Purchasers or such Notice
         Holder shall, on the basis of an opinion of nationally-recognized
         counsel experienced in such matters, determine to be required to be
         included therein by applicable law and make any required filings of
         such Prospectus supplement or such post-effective amendment; provided,
         that the Company shall not be required to take any actions under this
         Section 3(e) that are not, in the reasonable opinion of counsel for the
         Company, in compliance with applicable law.

                  (f)      As promptly as practicable furnish to each Notice
         Holder and the Initial Purchasers, upon their request and without
         charge, at least one (1) conformed copy of the Registration Statement
         and any amendment thereto, including financial statements but excluding
         schedules, all documents incorporated or deemed to be incorporated
         therein by reference and all exhibits (unless requested in writing to
         the Company by such Notice Holder or the Initial Purchasers, as the
         case may be).

                  (g)      During the Effectiveness Period, deliver to each
         Notice Holder in connection with any sale of Registrable Securities
         pursuant to a Registration Statement, without charge, as many copies of
         the Prospectus or Prospectuses relating to such Registrable Securities
         (including each preliminary prospectus) and any amendment or supplement
         thereto as such Notice Holder may reasonably request; and the Company
         hereby consents (except during such periods that a Deferral Notice is
         outstanding and has not been revoked) to the use of such Prospectus or
         each amendment or supplement

                                       9
<PAGE>

         thereto by each Notice Holder in connection with any offering and sale
         of the Registrable Securities covered by such Prospectus or any
         amendment or supplement thereto in the manner set forth therein.

                  (h)      Prior to any public offering of the Registrable
         Securities pursuant to the Shelf Registration Statement, use all
         reasonable efforts to register or qualify or cooperate with the Notice
         Holders in connection with the registration or qualification (or
         exemption from such registration or qualification) of such Registrable
         Securities for offer and sale under the securities or Blue Sky laws of
         such jurisdictions within the United States as any Notice Holder
         reasonably requests in writing (which request may be included in the
         Notice and Questionnaire); prior to any public offering of the
         Registrable Securities pursuant to the Shelf Registration Statement,
         use all reasonable efforts to keep each such registration or
         qualification (or exemption therefrom) effective during the
         Effectiveness Period in connection with such Notice Holder's offer and
         sale of Registrable Securities pursuant to such registration or
         qualification (or exemption therefrom) and do any and all other acts or
         things necessary or advisable to enable the disposition in such
         jurisdictions of such Registrable Securities in the manner set forth in
         the relevant Registration Statement and the related Prospectus;
         provided, that the Company will not be required to (i) qualify as a
         foreign corporation or as a dealer in securities in any jurisdiction
         where it would not otherwise be required to qualify but for this
         Agreement or (ii) take any action that would subject it to general
         service of process in suits or to taxation in any such jurisdiction
         where it is not then so subject.

                  (i)      Upon (A) the issuance by the SEC of a stop order
         suspending the effectiveness of the Shelf Registration Statement or the
         initiation of proceedings with respect to the Shelf Registration
         Statement under Section 8(d) or 8(e) of the Securities Act, (B) the
         occurrence of any event or the existence of any fact (a "Material
         Event") as a result of which any Registration Statement shall contain
         any untrue statement of a material fact or omit to state any material
         fact required to be stated therein or necessary to make the statements
         therein not misleading, or any Prospectus shall contain any untrue
         statement of a material fact or omit to state any material fact
         required to be stated therein or necessary to make the statements
         therein, in the light of the circumstances under which they were made,
         not misleading, or (C) the occurrence or existence of any pending
         corporate development that, in the discretion of the Company, makes it
         appropriate to suspend the availability of the Shelf Registration
         Statement and the related Prospectus, (i) in the case of clause (B)
         above, subject to the next sentence, as promptly as practicable prepare
         and file a post-effective amendment to such Registration Statement or a
         supplement to the related Prospectus or any document incorporated
         therein by reference or file any other required document that would be
         incorporated by reference into such Registration Statement and
         Prospectus so that such Registration Statement does not contain any
         untrue statement of a material fact or omit to state any material fact
         required to be stated therein or necessary to make the statements
         therein not misleading, and such Prospectus does not contain any untrue
         statement of a material fact or omit to state any material fact
         required to be stated therein or necessary to make the statements
         therein, in the light of the circumstances under which they were made,
         not misleading, as thereafter delivered to the purchasers of the
         Registrable Securities being sold thereunder, and, in the case of a
         post-effective amendment to a Registration Statement, subject to the
         next

                                       10
<PAGE>

         sentence, use all reasonable efforts to cause it to be declared
         effective as promptly as is reasonably practicable, and (ii) give
         notice to the Notice Holders that the availability of the Shelf
         Registration Statement is suspended (a "Deferral Notice") and, upon
         receipt of any Deferral Notice, each Notice Holder agrees not to sell
         any Registrable Securities pursuant to the Registration Statement until
         such Notice Holder's receipt of copies of the supplemented or amended
         Prospectus provided for in clause (i) above, or until it is advised in
         writing by the Company that the Prospectus may be used, and has
         received copies of any additional or supplemental filings that are
         incorporated or deemed incorporated by reference in such Prospectus.
         The Company will use all reasonable efforts to ensure that the use of
         the Prospectus may be resumed (x) in the case of clause (A) above, as
         promptly as is practicable, (y) in the case of clause (B) above, as
         soon as, in the sole judgment of the Company, public disclosure of such
         Material Event would not be prejudicial to or contrary to the interests
         of the Company or, if necessary to avoid unreasonable burden or
         expense, as soon as reasonably practicable thereafter and (z) in the
         case of clause (C) above, as soon as, in the discretion of the Company,
         such suspension is no longer appropriate. The period during which the
         availability of the Registration Statement and any Prospectus is
         suspended (the "Deferral Period") shall, without the Company incurring
         any obligation to pay liquidated damages pursuant to Section 2(e), not
         exceed thirty (30) days in any three (3) month period or ninety (90)
         days in any twelve (12) month period.

                  (j)      If reasonably requested in writing in connection with
         a disposition of Registrable Securities pursuant to a Registration
         Statement, make reasonably available for inspection during normal
         business hours by a representative for the Notice Holders of such
         Registrable Securities and any broker-dealers, attorneys and
         accountants retained by such Notice Holders, all relevant financial and
         other records, pertinent corporate documents and properties of the
         Company and its subsidiaries, and cause the appropriate executive
         officers, directors and designated employees of the Company and its
         subsidiaries to make reasonably available for inspection during normal
         business hours all relevant information reasonably requested by such
         representative for the Notice Holders or any such broker-dealers,
         attorneys or accountants in connection with such disposition, in each
         case as is customary for similar "due diligence" examinations;
         provided, however, that such persons shall first agree in writing with
         the Company that any information that is reasonably and in good faith
         designated by the Company in writing as confidential at the time of
         delivery of such information shall be kept confidential by such persons
         and shall be used solely for the purposes of exercising rights under
         this Agreement, unless (i) disclosure of such information is required
         by court or administrative order or is necessary to respond to
         inquiries of regulatory authorities, (ii) disclosure of such
         information is required by law (including any disclosure requirements
         pursuant to federal securities laws in connection with the filing of
         any Registration Statement or the use of any Prospectus referred to in
         this Agreement), (iii) such information becomes generally available to
         the public other than as a result of a disclosure or failure to
         safeguard by any such person or (iv) such information becomes available
         to any such person from a source other than the Company and such source
         is not bound by a confidentiality agreement; and provided further, that
         the foregoing inspection and information gathering shall, to the
         greatest extent possible, be coordinated

                                       11
<PAGE>

         on behalf of all the Notice Holders and the other parties entitled
         thereto by the counsel referred to in Section 5.

                  (k)      Comply with all applicable rules and regulations of
         the SEC and make generally available to its securityholders earning
         statements (which need not be audited) satisfying the provisions of
         Section 11(a) of the Securities Act and Rule 158 thereunder (or any
         similar rule promulgated under the Securities Act) no later than 45
         days after the end of any 12-month period (or 90 days after the end of
         any 12-month period if such period is a fiscal year), or such shorter
         period as required by the Securities Act and the Exchange Act and the
         rules and regulations thereunder, as in effect at any relevant time,
         commencing on the first day of the first fiscal quarter of the Company
         commencing after the effective date of a Registration Statement, which
         statements shall cover said 12-month periods.

                  (l)      Cooperate with each Notice Holder to facilitate the
         timely preparation and delivery of certificates representing
         Registrable Securities sold pursuant to a Registration Statement, and
         cause such Registrable Securities to be in such denominations as are
         permitted by the Indenture and registered in such names as such Notice
         Holder may request in writing at least two Business Days prior to any
         sale of such Registrable Securities.

                  (m)      Provide a CUSIP number for all Registrable Securities
         covered by each Registration Statement not later than the effective
         date of such Registration Statement and provide the Trustee for the
         Securities and the transfer agent for the Common Stock with printed
         certificates for the Registrable Securities that are in a form eligible
         for deposit with The Depository Trust Company.

                  (n)      Make a reasonable effort to provide such information
         as is required for any filings required to be made with the National
         Association of Securities Dealers, Inc.

                  (o)      Upon (i) the filing of the Initial Shelf Registration
         Statement and (ii) the effectiveness of the Initial Shelf Registration
         Statement, announce the same, in each case by release to Reuters
         Economic Services and Bloomberg Business News.

                  (p)      Enter into such customary agreements and take all
         such other reasonable necessary actions in connection therewith
         (including those reasonably requested by the holders of a majority of
         the Registrable Securities being sold) in order to expedite or
         facilitate disposition of such Registrable Securities.

                  (q)      Cause the Indenture to be qualified under the TIA not
         later than the effective date of any Registration Statement; and in
         connection therewith, cooperate with the Trustee to effect such changes
         to the Indenture as may be required for the Indenture to be so
         qualified in accordance with the terms of the TIA and execute, and use
         all reasonable efforts to cause the Trustee to execute, all documents
         as may be required to effect such changes, and all other forms and
         documents required to be filed with the SEC to enable the Indenture to
         be so qualified in a timely manner.

                                       12
<PAGE>

                  Section 4. Holder's Obligations. Each Holder agrees, by
acquisition of the Registrable Securities, that no Holder of Registrable
Securities shall be entitled to sell any of such Registrable Securities pursuant
to a Registration Statement or to receive a Prospectus relating thereto, unless
such Holder has furnished the Company with a Notice and Questionnaire as
required pursuant to Section 2(d) hereof (including the information required to
be included in such Notice and Questionnaire) and the information set forth in
the next sentence. Each Notice Holder agrees promptly to furnish to the Company
all information required to be disclosed in order to make the information
previously furnished to the Company by such Notice Holder not misleading and any
other information regarding such Notice Holder and the distribution of such
Registrable Securities as may be required to be disclosed in the Registration
Statement under applicable law.

                  Section 5. Registration Expenses. The Company shall bear all
fees and expenses incurred in connection with the performance by the Company of
its obligations under Sections 2 and 3 of this Agreement whether or not any of
the Registration Statements are declared effective. Such fees and expenses shall
include, without limitation, (i) all registration and filing fees (including,
without limitation, fees and expenses (x) with respect to filings required to be
made with the National Association of Securities Dealers, Inc. and (y) of
compliance with federal and state securities or Blue Sky laws (including,
without limitation, reasonable fees and disbursements of the counsel specified
in the next sentence in connection with Blue Sky qualifications of the
Registrable Securities under the laws of such jurisdictions as the Notice
Holders of a majority of the Registrable Securities being sold pursuant to a
Registration Statement may designate), (ii) printing expenses (including,
without limitation, expenses of printing certificates for Registrable Securities
in a form eligible for deposit with The Depository Trust Company), (iii)
duplication expenses relating to copies of any Registration Statement or
Prospectus delivered to any Holders hereunder, (iv) fees and disbursements of
counsel for the Company in connection with the Shelf Registration Statement, and
(v) reasonable fees and disbursements of the Trustee and its counsel and of the
registrar and transfer agent for the Common Stock. In addition, the Company
shall bear or reimburse the Notice Holders for the reasonable fees and
disbursements of one firm of legal counsel for the Holders, which shall
initially be Shearman & Sterling LLP, but which may, upon the written consent of
the Initial Purchasers (which shall not be unreasonably withheld), be another
nationally recognized law firm experienced in securities law matters designated
by the Company. In addition, the Company shall pay the internal expenses of the
Company (including, without limitation, all salaries and expenses of officers
and employees performing legal or accounting duties), the expense of any annual
audit, the fees and expenses incurred in connection with the listing of the
Registrable Securities on any securities exchange on which similar securities of
the Company are then listed and the fees and expenses of any person, including
special experts, retained by the Company.

                  Section 6. Indemnification; Contribution. (a) The Company
agrees to indemnify and hold harmless the Initial Purchasers and each holder of
Registrable Securities and each person, if any, who controls the Initial
Purchasers or any holder of Registrable Securities within the meaning of either
Section 15 of the Securities Act or Section 20 of the Exchange Act, as follows:

                                       13
<PAGE>

                  (i)      against any and all loss, liability, claim, damage
         and expense whatsoever, as incurred, arising out of any untrue
         statement or alleged untrue statement of a material fact contained in
         the Registration Statement (or any amendment thereto), or the omission
         or alleged omission therefrom of a material fact necessary in order to
         make the statements therein, in light of the circumstances under which
         they were made, not misleading or arising out of any untrue statement
         or alleged untrue statement of a material fact included in any
         preliminary prospectus or the Prospectus (or any amendment or
         supplement thereto), or the omission or alleged omission therefrom of a
         material fact necessary in order to make the statements therein, in the
         light of the circumstances under which they were made, not misleading;

                  (ii)     against any and all loss, liability, claim, damage
         and expense whatsoever, as incurred, to the extent of the aggregate
         amount paid in settlement of any litigation, or any investigation or
         proceeding by any governmental agency or body, commenced or threatened,
         or of any claim whatsoever based upon any such untrue statement or
         omission, or any such alleged untrue statement or omission, provided
         that (subject to Section 6(d) below) any such settlement is effected
         with the prior written consent of the Company; and

                  (iii)    against any and all expense whatsoever, as incurred
         (including the fees and disbursements of counsel), reasonably incurred
         in investigating, preparing or defending against any litigation, or any
         investigation or proceeding by any governmental agency or body,
         commenced or threatened, or any claim whatsoever based upon any such
         untrue statement or omission, or any such alleged untrue statement or
         omission, to the extent that any such expense is not paid under (i) or
         (ii) above;

provided, however, that this indemnity shall not apply to any loss, liability,
claim, damage or expense to the extent arising out of any untrue statement or
omission or alleged untrue statement or omission made in reliance upon and in
conformity with information furnished to the Company by each and any Initial
Purchaser or such holder of Registrable Securities (which also acknowledges the
indemnity provisions herein) and each person, if any, who controls any such
Initial Purchaser or any such holder of Registrable Securities expressly for use
in the Registration Statement (or any amendment thereto), or any preliminary
prospectus or the Prospectus (or any amendment or supplement thereto).

                  (b)      In connection with any Shelf Registration in which a
holder, including, without limitation, each Initial Purchaser, of Registrable
Securities is participating, in furnishing information relating to such holder
of Registrable Securities to the Company expressly for use in such Registration
Statement, any preliminary prospectus, the Prospectus or any amendments or
supplements thereto, the holders of such Registrable Securities agree, severally
and not jointly, to indemnify and hold harmless each Initial Purchaser and each
person, if any, who controls any Initial Purchaser within the meaning of either
Section 15 of the Securities Act or Section 20 of the Exchange Act and the
Company, and each person, if any, who controls the Company within the meaning of
either such Section, against any and all loss, liability, claim, damage and
expense described in the indemnity contained in subsection (a) of this Section,
as incurred, but only with respect to untrue statements or omissions, or alleged
untrue statements or omissions, made in the Registration Statement (or any
amendment thereto), or any preliminary prospectus or the

                                       14
<PAGE>

Prospectus (or any amendment or supplement thereto) in reliance upon and in
conformity with written information furnished to the Company by such holder of
Registrable Securities (which also acknowledges the indemnity provisions herein)
and each person, if any, who controls any such holder of Registrable Securities
expressly for use in the Registration Statement (or any amendment thereto) or
such preliminary prospectus or the Prospectus (or any amendment or supplement
thereto).

                  Each of the Initial Purchasers agrees to indemnify and hold
harmless the Company, the holders of Registrable Securities, and each person, if
any, who controls the Company or any holder of Registrable Securities within the
meaning of either Section 15 of the Securities Act or Section 20 of the Exchange
Act against any and all loss, liability, claim, damage and expense described in
the indemnity contained in subsection (a) of this Section, as incurred, but only
with respect to untrue statements or omissions, or alleged untrue statements or
omissions, made in the Registration Statement (or any amendment thereto), or any
preliminary prospectus or the Prospectus (or any amendment or supplement
thereto) in reliance upon and in conformity with information furnished to the
Company by such Initial Purchaser expressly for use in the Registration
Statement (or any amendment thereto) or such preliminary prospectus or the
Prospectus (or any amendment or supplement thereto).

                  (c)      Each indemnified party shall give notice as promptly
as reasonably practicable to each indemnifying party of any action commenced
against it in respect of which indemnity may be sought hereunder, but failure to
so notify an indemnifying party shall not relieve such indemnifying party from
any liability hereunder to the extent it is not materially prejudiced as a
result thereof and in any event shall not relieve it from any liability which it
may have otherwise than on account of this indemnity agreement. An indemnifying
party may participate at its own expense in the defense of any such action;
provided, however, that counsel to the indemnifying party shall not (except with
the consent of the indemnified party) also be counsel to the indemnified party.
In no event shall the indemnifying parties be liable for fees and expenses of
more than one counsel (in addition to any local counsel) separate from their own
counsel for all indemnified parties in connection with any one action or
separate but similar or related actions in the same jurisdiction arising out of
the same general allegations or circumstances. No indemnifying party shall,
without the prior written consent of the indemnified parties, settle or
compromise or consent to the entry of any judgment with respect to any
litigation, or any investigation or proceeding by any governmental agency or
body, commenced or threatened, or any claim whatsoever in respect of which
indemnification or contribution could be sought under this Section 6 hereof
(whether or not the indemnified parties are actual or potential parties
thereto), unless such settlement, compromise or consent (i) includes an
unconditional release of each indemnified party from all liability arising out
of such litigation, investigation, proceeding or claim and (ii) does not include
a statement as to or an admission of fault, culpability or a failure to act by
or on behalf of any indemnified party.

                  (d)      If at any time an indemnified party shall have
requested an indemnifying party to reimburse the indemnified party for fees and
expenses of counsel, such indemnifying party agrees that it shall be liable for
any settlement of the nature contemplated by Section 6(a)(ii) effected without
its written consent if (i) such settlement is entered into more than 45 days
after receipt by such indemnifying party of aforesaid request, (ii) such
indemnifying party shall have received notice of the terms of such settlement at
least 30 days prior to such settlement

                                       15
<PAGE>

being entered into and (iii) such indemnifying party shall not have reimbursed
such indemnified party in accordance with such request for fees and expenses
prior to the date of such settlement.

                  (e)      If the indemnification provided for in this Section 6
is for any reason unavailable to or insufficient to hold harmless an indemnified
party in respect of any losses, liabilities, claims, damages or expenses
referred to therein, then each indemnifying party shall contribute to the
aggregate amount of such losses, liabilities, claims, damages and expenses
incurred by such indemnified party, as incurred, in such proportion as is
appropriate to reflect the relative fault of the indemnifying party or parties
on the one hand and of the indemnified party on the other hand in connection
with the statements or omissions which resulted in such losses, liabilities,
claims, damages or expenses, as well as any other relevant equitable
considerations.

                  The relative fault of the Company on the one hand and the
holders of the Registrable Securities or such Initial Purchaser on the other
hand shall be determined by reference to, among other things, whether any such
untrue or alleged untrue statement of a material fact or omission or alleged
omission to state a material fact relates to information supplied by the Company
or by the holder of the Registrable Securities or such Initial Purchaser and the
parties' relative intent, knowledge, access to information and opportunity to
correct or prevent such statement or omission.

                  The parties hereto agree that it would not be just and
equitable if contribution pursuant to this Section 6(e) were determined by pro
rata allocation or by any other method of allocation which does not take account
of the equitable considerations referred to above in this Section 6(e). The
aggregate amount of losses, liabilities, claims, damages, and expenses incurred
by an indemnified party and referred to above in this Section 6(e) shall be
deemed to include any legal or other expenses reasonably incurred by such
indemnified party in investigating, preparing or defending against any
litigation, or any investigation or proceeding by any governmental agency or
body, commenced or threatened, or any claim whatsoever based upon any such
untrue or alleged untrue statement or omission or alleged omission.

                  Notwithstanding the provisions of this Section 6, neither the
holder of any Registrable Securities nor any Initial Purchaser, shall be
required to indemnify or contribute any amount in excess of the amount by which
the total price at which the Registrable Securities sold by such holder of
Registrable Securities or unwritten by such Initial Purchaser, as the case may
be, and distributed to the public were offered to the public exceeds the amount
of any damages that such holder of Registrable Securities or such Initial
Purchaser have otherwise been required to pay by reason of such untrue or
alleged untrue statement or omission or alleged omission.

                  No person guilty of fraudulent misrepresentation (within the
meaning of Section 11(f) of the Securities Act) shall be entitled to
contribution from any person who was not guilty of such fraudulent
misrepresentation.

                  For purposes of this Section 6(e), each person, if any, who
controls any Initial Purchaser or any holder of Registrable Securities within
the meaning of Section 15 of the Securities Act or Section 20 of the Exchange
Act shall have the same rights to contribution as such Initial Purchaser or such
holder, and each person, if any, who controls the Company within

                                       16
<PAGE>

the meaning of Section 15 of the Securities Act or Section 20 of the Exchange
Act shall have the same rights to contribution as the Company.

                  Section 7. Information Requirements. The Company covenants
that, if at any time before the end of the Effectiveness Period the Company is
not subject to the reporting requirements of the Exchange Act, it will cooperate
with any Holder of Registrable Securities and take such further reasonable
action as any Holder of Registrable Securities may reasonably request in writing
(including, without limitation, making such reasonable representations as any
such Holder may reasonably request), all to the extent required from time to
time to enable such Holder to sell Registrable Securities without registration
under the Securities Act within the limitation of the exemptions provided by
Rule 144 and Rule 144A under the Securities Act and customarily taken in
connection with sales pursuant to such exemptions. Upon the written request of
any Holder of Registrable Securities, the Company shall deliver to such Holder a
written statement as to whether it has complied with such filing requirements,
unless such a statement has been included in the Company's most recent report
required to be filed and filed pursuant to Section 13 or Section 15(d) of
Exchange Act. Notwithstanding the foregoing, nothing in this Section 7 shall be
deemed to require the Company to register any of its securities (other than
Common Stock) under any section of the Exchange Act.

                  Section 8. Miscellaneous. (a) No Conflicting Agreements. The
Company is not, as of the date hereof, a party to, nor shall it, on or after the
date of this Agreement, enter into, any agreement with respect to its securities
that conflicts with the rights granted to the Holders of Registrable Securities
in this Agreement. The Company represents and warrants that the rights granted
to the Holders of Registrable Securities hereunder do not in any way conflict
with the rights granted to the holders of the Company's securities under any
other agreements.

                  (b)      Amendments and Waivers. The provisions of this
Agreement, including the provisions of this sentence, may not be amended,
modified or supplemented, and waivers or consents to departures from the
provisions hereof may not be given, unless the Company has obtained the written
consent of Holders of a majority of the then outstanding Underlying Common Stock
constituting Registrable Securities (with Holders of Securities deemed to be the
Holders, for purposes of this Section, of the number of outstanding shares of
Underlying Common Stock into which such Securities are or would be convertible
or exchangeable as of the date on which such consent is requested).
Notwithstanding the foregoing, a waiver or consent to depart from the provisions
hereof with respect to a matter that relates exclusively to the rights of
Holders of Registrable Securities whose securities are being sold pursuant to a
Registration Statement and that does not directly or indirectly affect the
rights of other Holders of Registrable Securities may be given by Holders of at
least a majority of the Registrable Securities being sold by such Holders
pursuant to such Registration Statement; provided, that the provisions of this
sentence may not be amended, modified, or supplemented except in accordance with
the provisions of the immediately preceding sentence. Each Holder of Registrable
Securities outstanding at the time of any such amendment, modification,
supplement, waiver or consent or thereafter shall be bound by any such
amendment, modification, supplement, waiver or consent effected pursuant to this
Section 8(b), whether or not any notice, writing or marking indicating such
amendment, modification, supplement, waiver or consent appears on the
Registrable Securities or is delivered to such Holder.

                                       17
<PAGE>

                  (c)      Notices. All notices and other communications
provided for or permitted hereunder shall be made in writing by hand delivery,
by telecopier, by courier guaranteeing overnight delivery or by first-class
mail, return receipt requested, and shall be deemed given (i) when made, if made
by hand delivery, (ii) upon confirmation, if made by telecopier, (iii) one (1)
Business Day after being deposited with such courier, if made by overnight
courier or (iv) on the date indicated on the notice of receipt, if made by
first-class mail, to the parties as follows:

                  (w)      if to a Holder of Registrable Securities that is not
         a Notice Holder, at the address for such Holder then appearing in the
         Registrar (as defined in the Indenture);

                  (x)      if to a Notice Holder, at the most current address
         given by such Holder to the Company in a Notice and Questionnaire or
         any amendment thereto;

                  (y)      if to the Company, to:

                  OSI Pharmaceuticals, Inc.
                  58 South Service Road, Suite 110
                  Melville, NY 11747
                  Attention: Barbara Wood, General Counsel

                  Telephone No.: (631) 962-2000
                  Telecopy No.: (631) 752-3880

                  and

                  (z)      if to the Initial Purchasers, to:

                  Merrill Lynch & Co.
                  Merrill Lynch, Pierce, Fenner & Smith Incorporated
                  Morgan Stanley & Co. Incorporated

                  c/o      Merrill Lynch & Co.
                           Merrill Lynch, Pierce, Fenner & Smith
                                            Incorporated
                           4 World Financial Center
                           New York, New York 10080
                           Attention: Equity Capital Markets

or to such other address as such person may have furnished to the other persons
identified in this Section 8(c) in writing in accordance herewith.

                  (d)      Approval of Holders. Whenever the consent or approval
of Holders of a specified percentage of Registrable Securities is required
hereunder, the Registrable Securities held by the Company or its Affiliates
(other than the Initial Purchasers or subsequent Holders of Registrable
Securities if such subsequent Holders are deemed to be such Affiliates solely by
reason of their holdings of such Registrable Securities) shall not be counted in
determining whether such consent or approval was given by the Holders of such
required percentage.

                                       18
<PAGE>

                  (e)      Successors and Assigns. Any person who purchases any
Registrable Securities from an Initial Purchaser shall be deemed, for purposes
of this Agreement, to be an assignee of such Initial Purchaser. This Agreement
shall inure to the benefit of and be binding upon the successors and assigns of
each of the parties and shall inure to the benefit of and be binding upon each
Holder of any Registrable Securities.

                  (f)      Counterparts. This Agreement may be executed in any
number of counterparts and by the parties hereto in separate counterparts, each
of which when so executed shall be deemed to be original and all of which taken
together shall constitute one and the same agreement.

                  (g)      Headings. The headings in this Agreement are for
convenience of reference only and shall not limit or otherwise affect the
meaning hereof.

                  (h)      Governing Law. This agreement shall be governed by
and construed in accordance with the laws of the State of New York, without
regard to conflicts of laws principles thereof.

                  (i)      Severability. If any term, provision, covenant or
restriction of this Agreement is held to be invalid, illegal, void or
unenforceable, the remainder of the terms, provisions, covenants and
restrictions set forth herein shall remain in full force and effect and shall in
no way be affected, impaired or invalidated thereby, and the parties hereto
shall use their best efforts to find and employ an alternative means to achieve
the same or substantially the same result as that contemplated by such term,
provision, covenant or restriction, it being intended that all of the rights and
privileges of the parties shall be enforceable to the fullest extent permitted
by law.

                  (j)      Entire Agreement. This Agreement is intended by the
parties as a final expression of their agreement and is intended to be a
complete and exclusive statement of the agreement and understanding of the
parties hereto in respect of the subject matter contained herein and the
registration rights granted by the Company with respect to the Registrable
Securities. Except as provided in the Purchase Agreement, there are no
restrictions, promises, warranties or undertakings, other than those set forth
or referred to herein, with respect to the registration rights granted by the
Company with respect to the Registrable Securities. This Agreement supersedes
all prior agreements and undertakings among the parties with respect to such
registration rights.

                  (k)      Termination. This Agreement and the obligations of
the parties hereunder shall terminate upon the expiration of the Effectiveness
Period, except for any liabilities or obligations under Sections 4, 5 or 6
hereof and the obligations to make payments of and provide for liquidated
damages under Section 2(e) hereof to the extent such damages accrue prior to the
end of the Effectiveness Period, each of which shall remain in effect in
accordance with its terms.

                [REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                       19
<PAGE>

                  IN WITNESS WHEREOF, the parties have executed this
Registration Rights Agreement as of the date first written above.

                                                    OSI PHARMACEUTICALS, INC.

                                                    By:   /s/
                                                          ______________________
                                                          Name:
                                                          Title:

CONFIRMED AND ACCEPTED,
         as of the date first above written:

MERRILL LYNCH & CO.
MERRILL LYNCH, PIERCE, FENNER & SMITH
            INCORPORATED
MORGAN STANLEY & CO. INCORPORATED

By: MERRILL LYNCH, PIERCE, FENNER & SMITH
               INCORPORATED

By: /s/
    _______________________________________
    Name:
    Title: