<PAGE>
                                                                     EXHIBIT 4.1


                            OSI PHARMACEUTICALS, INC.

                       NON-QUALIFIED STOCK OPTION PLAN FOR
                               FORMER EMPLOYEES OF
                              GILEAD SCIENCES, INC.

1.    Purpose

            OSI Pharmaceuticals, Inc. (the "Company") has acquired certain of
      the assets of Gilead Sciences, Inc. ("Gilead"). In connection therewith,
      the Company has adopted this Non-Qualified Stock Option Plan for Former
      Employees of Gilead Sciences, Inc. (the "Plan") as an incentive to induce
      certain former employees of Gilead to accept employment with, or become
      associated with, the Company or a parent or subsidiary of the Company, and
      to encourage them to acquire a proprietary interest in the Company through
      the ownership of common stock, par value $.01 per share (the "Common
      Stock"), of the Company. Such ownership will provide them with a more
      direct stake in the future welfare of the Company. No option granted under
      the Plan shall be considered an "incentive stock option" as defined in
      Section 422 of the Internal Revenue Code of 1986, as amended (the "Code").

            As used herein, the term "parent" or "subsidiary" shall mean any
      present or future corporation which is or would be a "parent corporation"
      or "subsidiary corporation" of the Company as the term is defined in
      Section 424 of the Code (determined as if the Company were the employer
      corporation).

2.    Administration of the Plan

            The Plan shall be administered by a committee (the "Committee") as
      appointed from time to time by the Board of Directors of the Company,
      which may be the Compensation Committee of the Board of Directors. Except
      as otherwise specifically provided herein, no person, other than members
      of the Committee, shall have any discretion as to decisions regarding the
      Plan. The Company may engage a third party to administer routine matters
      under the Plan, such as establishing and maintaining accounts for Plan
      participants and facilitating transactions by participants pursuant to the
      Plan.

            In administering the Plan, the Committee may adopt rules and
      regulations for carrying out the Plan. The interpretations and decisions
      made by the Committee with regard to any question arising under the Plan
      shall be final and conclusive on all persons participating or eligible to
      participate in the Plan. Subject to the provisions of the Plan, the
      Committee shall determine the terms of all options granted pursuant to the
      Plan, including, but not limited to, the persons to whom, and the time or
      times at which, grants shall be made, the number of shares to be covered
      by each option, the duration of options, the exercisability of options,
      and the option price.



3.    Shares of Stock Subject to the Plan

            Except as provided in paragraphs 6(h), 6(i) and 7 hereof, the number
      of shares that may be issued or transferred pursuant to the exercise of
      options granted under the Plan shall not exceed 750,249 shares of Common
      Stock. Such shares may be authorized and unissued shares or previously
      issued shares acquired or to be acquired by the Company and held in
      treasury.
<PAGE>
4.    Eligibility

            Options may be granted only to directors, officer, employees and
      consultants who are former employees of Gilead.

5.    Granting of options

            No options pursuant to this Plan may be granted after ten years from
      the effective date. The date of the grant of any option shall be the date
      on which the Committee authorizes the grant of such option.

6.    Options

            Options shall be evidenced by stock option agreements in such form,
      consistent with the Plan, as the Committee shall approve from time to
      time, which agreements need not be identical and shall be subject to the
      following terms and conditions:

                  (a) Option Price. The purchase price under each option shall
            be specified by the Committee, but shall in no case be less than the
            greater of the Fair Market Value of the Common Stock at the time the
            option is granted and the par value of such Common Stock.

                  (b) Medium and Time of Payment. Stock purchased pursuant to
            the exercise of an option shall at the time of purchase be paid for
            in full in cash, or, upon conditions established by the Committee,
            by delivery of shares of Common Stock owned by the recipient. If
            payment is made by the delivery of shares, the value of the shares
            delivered shall be the Fair Market Value of such shares on the date
            of exercise of the option. In addition, unless otherwise provided by
            the Committee, an "in the money" option may be exercised on a
            "cashless" basis in exchange for the issuance to the optionee (or
            other person entitled to exercise the option) of the largest whole
            number of shares having an aggregate value equal to the value of
            such option on the date of exercise. For this purpose, the value of
            the shares delivered by the Company and the value of the option
            being exercised shall be determined based on the Fair Market Value
            of the Common Stock on the date of exercise of the option. Upon
            receipt of payment and such documentation as the Company may deem
            necessary to establish compliance with the Securities Act of 1933,
            as amended (the "Securities Act"), the Company shall, without stock
            transfer tax to the optionee or other person entitled to exercise
            the option, deliver to the person exercising the option a
            certificate or certificates for such shares. It shall be a condition
            to the performance of the Company's obligation to issue or transfer
            Common Stock upon exercise of an option or options that the optionee
            pay, or make provision satisfactory to the Company for the payment
            of, any taxes (other than stock transfer taxes) the Company is
            obligated to collect with respect to the issue or transfer of Common
            Stock upon such exercise, including any federal, state, or local
            withholding taxes.

                  (c) Waiting Period. The waiting period and time for exercising
            an option shall be prescribed by the Committee in each particular
            case; provided, however, that no option may be exercised after 10
            years from the date it is granted.

                  (d) Rights as a Stockholder. A recipient of options shall have
            no rights as a stockholder with respect to any shares issuable or
            transferable upon exercise thereof until the date a stock
            certificate is issued to him for such shares. Except as otherwise
            expressly provided in the Plan, no adjustment shall be made for
            dividends or other rights for which the record date is prior to the
            date such stock certificate is issued.
<PAGE>
                  (e) Non-Assignability of Options. Except as may otherwise be
            specifically provided by the Committee, no option shall be
            assignable or transferable by the recipient except by will or by the
            laws of descent and distribution. During the lifetime of a
            recipient, except as may otherwise be specifically provided by the
            Committee, options shall be exercisable only by such recipient. If
            the Committee approves provisions in any particular case allowing
            for assignment or transfer of an option, then such option will
            nonetheless be subject to a six-month holding period commencing on
            the date of grant during which period the recipient will not be
            permitted to assign or transfer such option, unless the Committee
            further specifically provides for the assignability or
            transferability of such option during this period. See paragraph 8
            hereof for restrictions on sale of shares.

                  (f) Effect of Termination of Employment. If a recipient's
            employment (or service as an officer, director or consultant) shall
            terminate for any reason, other than death or Retirement (as defined
            below), the right of the recipient to exercise any option otherwise
            exercisable on the date of such termination shall expire unless such
            right is exercised within a period of 90 days after the date of such
            termination. The term "Retirement" shall mean the voluntary
            termination of employment (or service as an officer, director or
            consultant) by a recipient who has attained the age of 55 and who
            has completed at least five years of service with the Company. If a
            recipient's employment (or service as an officer, director or
            consultant) shall terminate because of death or Retirement, the
            right of the recipient to exercise any option otherwise exercisable
            on the date of such termination shall be unaffected by such
            termination and shall continue until the normal expiration of such
            option. Option rights shall not be affected by any change of
            employment as long as the recipient continues to be employed by
            either the Company or a parent or subsidiary of the Company. In no
            event, however, shall an option be exercisable after the expiration
            of its original term as determined by the Committee pursuant to
            subparagraph 6(c) above. The Committee may, if it determines that to
            do so would be in the Company's best interests, provide in a
            specific case or cases for the exercise of options which would
            otherwise terminate upon termination of employment with the Company
            for any reason, upon such terms and conditions as the Committee
            determines to be appropriate. Nothing in the Plan or in any option
            agreement shall confer any right to continue in the employ of the
            Company or any parent or subsidiary of the Company or interfere in
            any way with the right of the Company or any parent or subsidiary of
            the Company to terminate the employment of a recipient at any time.

                  (g) Leave of Absence. In the case of a recipient on an
            approved leave of absence, the Committee may, if it determines that
            to do so would be in the best interests of the Company, provide in a
            specific case for continuation of options during such leave of
            absence, such continuation to be on such terms and conditions as the
            Committee determines to be appropriate, except that in no event
            shall an option be exercisable after 10 years from the date it is
            granted.

                  (h) Recapitalization. In the event that dividends payable in
            Common Stock during any fiscal year of the Company exceed in the
            aggregate five percent of the Common Stock issued and outstanding at
            the beginning of the year, or in the event there is during any
            fiscal year of the Company one or more splits, subdivisions, or
            combinations of shares of Common Stock resulting in an increase or
            decrease by more than five percent of the shares outstanding at the
            beginning of the year, the number of shares available under the Plan
            shall be increased or decreased proportionately, as the case may be,
            and the number of shares deliverable upon the exercise thereafter of
            any options theretofore granted shall be increased or decreased
            proportionately, as the case may be, without change in the aggregate
            purchase price. Common Stock dividends, splits, subdivisions, or
            combinations during any fiscal year that do not exceed in the
            aggregate five percent of the Common Stock issued and outstanding at
            the beginning of such year shall be ignored for purposes of the
            Plan. All adjustments shall be made as of the day such action
            necessitating such adjustment becomes effective.
<PAGE>
                  (i) Sale or Reorganization. In case the Company is merged or
            consolidated with another corporation, or in case the property or
            stock of the Company is acquired by another corporation, or in case
            of a separation, reorganization, or liquidation of the Company, the
            Board of Directors of the Company, or the board of directors of any
            corporation assuming the obligations of the Company hereunder, shall
            either (i) make appropriate provisions for the protection of any
            outstanding options by the substitution on an equitable basis of
            appropriate stock of the Company, or appropriate stock or options of
            the merged, consolidated, or otherwise reorganized corporation, or
            (ii) give written notice to optionees that their options, which will
            become immediately exercisable notwithstanding any waiting period
            otherwise prescribed by the Committee, must be exercised within 30
            days of the date of such notice or they will be terminated.

                  (j) General Restrictions. Each option granted under the Plan
            shall be subject to the requirement that, if at any time the Board
            of Directors shall determine, in its discretion, that the listing,
            registration, or qualification of the shares issuable or
            transferable upon exercise thereof upon any securities exchange or
            under any state or federal law, or the consent or approval of any
            governmental regulatory body is necessary or desirable as a
            condition of, or in connection with, the granting of such option or
            the issue, transfer, or purchase of shares thereunder, such option
            may not be exercised in whole or in part unless such listing,
            registration, qualification, consent, or approval shall have been
            effected or obtained free of any conditions not acceptable to the
            Board of Directors.

                  The Company shall not be obligated to sell or issue any shares
            of Common Stock in any manner in contravention of the Securities
            Act, the Securities Exchange Act of 1934, as amended (the "Exchange
            Act"), the rules and regulations of the Securities and Exchange
            Commission, any state securities law, the rules and regulations
            promulgated thereunder or the rules and regulations of any
            securities exchange or over the counter market on which the Common
            Stock is listed or in which it is included for quotation. The Board
            of Directors may, in connection with the granting of each option,
            require the individual to whom the option is to be granted to enter
            into an agreement with the Company stating that as a condition
            precedent to each exercise of the option, in whole or in part, he
            shall, if then required by the Company, represent to the Company in
            writing that such exercise is for investment only and not with a
            view to distribution, and also setting forth such other terms and
            conditions as the Committee may prescribe. Such agreements may also,
            in the discretion of the Committee, contain provisions requiring the
            forfeiture of any options granted and/or Common Stock held, in the
            event of the termination of employment or association, as the case
            may be, of the optionee with the Company. Upon any forfeiture of
            Common Stock pursuant to an agreement authorized by the preceding
            sentence, the Company shall pay consideration for such Common Stock
            to the optionee, pursuant to any such agreement, without interest
            thereon.

                  (k) "Fair Market Value." Fair Market Value for all purposes
            under the Plan shall mean the closing price of shares of Common
            Stock, as reported in The Wall Street Journal, in the NASDAQ
            National Market Issues or similar successor consolidated
            transactions reports (or a similar consolidated transactions report
            for the exchange on which the shares of Common Stock are then
            trading) for the relevant date, or if no sales of shares of Common
            Stock were made on such date, the average of the high and low sale
            prices of shares as reported in such composite transaction report
            for the preceding day on which sales of shares were made. If the
            shares are not listed on a national securities exchange or included
            for quotation in the NASDAQ National Market System at the time Fair
            Market Value is to be determined, then Fair Market Value shall be
            determined by the Committee in good faith pursuant to such method as
            to the Committee deems appropriate and equitable. Under no
            circumstances shall the Fair Market Value of a share of Common Stock
            be less than its par value.
<PAGE>
7.    Termination and Amendment of the Plan

            The Board of Directors or the Committee shall have the right to
      amend, suspend, or terminate the Plan at any time; provided, however, that
      no such action shall affect or in any way impair the rights of a recipient
      under any option right theretofore granted under the Plan; and, provided,
      further, that unless first duly approved by the stockholders of the
      Company entitled to vote thereon at a meeting (which may be the annual
      meeting) duly called and held for such purpose, except as provided in
      subparagraphs 6(h) and 6(i), no amendment or change shall be made in the
      Plan increasing the total number of shares which may be issued or
      transferred under the Plan, materially increasing the benefits to Plan
      participants or modifying the requirements as to eligibility for
      participation in the Plan.

8.    Restriction on Sale of Shares

            No stock acquired by an optionee upon exercise of an option granted
      hereunder may be disposed of by the optionee (or other person eligible to
      exercise the option) within six months from the date such option was
      granted, unless otherwise provided by the Committee.

9.    Effective Date of the Plan

            This Plan shall become effective on January 1, 2002 (the "Effective
      Date"). The Plan shall terminate after ten years from the Effective Date,
      or on such earlier date as the Board of Directors or the Committee may
      determine. Any option outstanding at the termination date shall remain
      outstanding until it has either expired or has been exercised.

10.   Compliance with Rule 16b-3

            With respect to persons subject to Section 16 of the Exchange Act,
      transactions under this Plan are intended to comply with all applicable
      conditions of Rule 16b-3 or its successors. To the extent any provision of
      the Plan or action by the Committee (or any other person on behalf of the
      Committee or the Company) fails to so comply, it shall be deemed null and
      void, to the extent permitted by law and deemed advisable by the
      Committee.

11.    Options granted to employees and directors of any subsidiary in the UK

             In addition to the provisions of paragraphs 1 to 10 (inclusive)
       above, the provisions of this paragraph 11 shall apply as herein set out
       to options granted to employees and directors of any subsidiary in the
       United Kingdom. The provisions of this paragraph 11 enable the Plan to be
       used in a tax efficient manner in the United Kingdom.

       (a) In this paragraph 11 the following terms have the meanings ascribed
           to them:

             "Election" means an election in the form envisaged in Paragraph
             3B(1) of Schedule 1 to SSCBA and acceptable to the UK Subsidiary to
             the effect that any Secondary NIC arising on the exercise,
             assignment or release of a UK Option shall be the liability of the
             recipient and not the liability of the UK Subsidiary

             "Independent Transfer Agent" means any person (other than the
             Company or any company affiliated with the Company or any
             individual affiliated with any such company) who is registered as a
             broker-dealer with the U.S. Securities and Exchange Commission and
             who is thereby able to sell and transfer shares in the Company on
             behalf of the Optionholder
<PAGE>
             "Optionholder" means an employee or director of the UK Subsidiary
             who is the holder of a UK Option

             "Secondary NIC" means secondary national insurance contributions
             as defined in the SSCBA

             "SSCBA" means the Social Security Contributions and Benefits Act
             1992 of the United Kingdom

             "UK Option" means an option granted to an employee of the UK
             Subsidiary

             "UK Subsidiary" means OSI Pharmaceuticals (UK) Limited (a company
             incorporated in England under company number 1709877) and any other
             UK Subsidiary of the Company from time to time.

                  (b) To the extent that it is lawful to do so, a UK Option may
             be granted subject to a condition that any liability of the UK
             Subsidiary (as employer or former employer of the relevant
             Optionholder) to pay Secondary NIC in respect of the exercise,
             assignment or release of that UK Option shall be the liability of
             the relevant Optionholder and payable by that Optionholder and that
             the Optionholder shall not be entitled to exercise the UK Option
             until he has entered into an Election to that effect when required
             to do so by the UK Subsidiary provided that the Committee may in
             its discretion at any time or times release the Optionholder from
             this liability or reduce his liability thereunder unless that
             Election has been entered into between the UK Subsidiary and that
             Optionholder and that Election (or the legislation which provides
             for such an Election to be effective) does not allow for such an
             Election to be subsequently varied.

                  (c) If a UK Option is granted subject to the condition
             referred to in paragraph 11(b) above then the Optionholder shall by
             completing the Election grant to the UK Subsidiary (as employer or
             former employer of the relevant Optionholder) the irrevocable
             authority, as agent of the Optionholder and on his behalf, to
             appoint an Independent Transfer Agent, to act as agent of the
             Optionholder and on his behalf, to sell or procure the sale of
             sufficient of the Stock subject to the UK Option and remit the net
             sale proceeds to the UK Subsidiary so that the net proceeds payable
             to the UK Subsidiary are so far as possible equal to but not less
             than the amount of the Secondary NIC for which the Optionholder is
             liable under the terms of the Election and the UK Subsidiary shall
             account to the Optionholder for any balance.

                        No Stock shall be allotted or transferred to the
             Optionholder by the Company until the UK Subsidiary has received an
             amount in cash equal to the amount of the Secondary NIC for which
             the Optionholder is liable under the terms of the Election.


                  (d) If a UK Option is exercised and the Optionholder is liable
             to tax duties or other amounts on such exercise and the UK
             Subsidiary (as his employer or former employer) is liable to make a
             payment to the appropriate authorities on account of that liability
             then the Optionholder shall by having completed the Option
             Agreement grant to the UK Subsidiary (as employer or former
             employer of the relevant Optionholder) the irrevocable authority,
             as agent of the Optionholder and on his behalf, to appoint an
             Independent Transfer Agent, to act as agent of the Optionholder and
             on his behalf, to sell or procure the sale of sufficient of the
             Shares subject to the UK Option and remit the net sale proceeds to
             the UK Subsidiary so that the net proceeds payable to the UK
             Subsidiary are so far as possible equal to but not less than the
             amount payable to the appropriate authorities and the UK Subsidiary
             shall account to the Optionholder for any balance.

                        No Stock shall be allotted or transferred to the
             Optionholder by the Company until the UK Subsidiary has received an
             amount in cash equal to the amount of any liability of the UK
             Subsidiary referred to in this paragraph (d).