<PAGE>

                                                                   Exhibit 10.16

                 FIRST AMENDMENT TO CERTAIN OPERATIVE AGREEMENTS
                               (AOR Trust 1997-1)

         THIS FIRST AMENDMENT TO CERTAIN OPERATIVE AGREEMENTS executed on
October 18, 2002 (this "Amendment") is by and among AOR synthetic REAL ESTATE,
INC., a Delaware corporation (the "Lessee" or a "Tranche A Guarantor"); US
ONCOLOGY, INC., a Delaware corporation, (the "Guarantor" or a "Tranche A
Guarantor"); the various entities which are parties hereto from time to time as
guarantors of the Tranche A Loans (subject to the definition of Tranche A
Guarantors in Appendix A to the Participation Agreement (hereinafter defined),
individually a "Tranche A Guarantor" and collectively, the "Tranche A
Guarantors"); WELLS FARGO BANK NORTHWEST, NATIONAL ASSOCIATION (as successor to
First Security Bank, National Association), a national banking association, not
individually (in its individual capacity, the "Trust Company"), except as
expressly stated herein, but solely as the Owner Trustee under the AOR Trust
1997-1 (the "Owner Trustee", the "Borrower" or the "Lessor"); the various banks
and other lending institutions which are parties to the Participation Agreement
from time to time as lenders (subject to the definition of Lenders in Appendix A
to the Participation Agreement, individually, a "Lender" and collectively, the
"Lenders"); WACHOVIA BANK, NATIONAL ASSOCIATION (as successor in interest to
First Union National Bank), a national banking association, as the agent for the
Lenders and respecting the Security Documents, as the agent for the Lenders and
the Holders, to the extent of their interests (in such capacity, the "Agent");
and the various banks and other lending institutions which are parties to the
Participation Agreement from time to time as holders of certificates issued with
respect to the AOR Trust 1997-1 (subject to the definition of Holders in
Appendix A to the Participation Agreement, individually, a "Holder" and
collectively, the "Holders").

                               W I T N E S S E T H

         WHEREAS, the parties to this Amendment are parties to the Amended and
Restated Participation Agreement dated as of February 1, 2002 (as amended,
modified, extended, supplemented, restated and/or replaced from time to time,
the "Participation Agreement");

         WHEREAS, the parties to this Amendment desire to provide the Lessee
with an option (subject to the terms of this Amendment) to amend various
provisions of the Operative Agreements on or prior to December 31, 2003 in order
to, among other things, (a) allow the Lessee (i) to move certain items of
personal property which comprise portions of the Properties to various locations
which locations are either owned or leased by one or more Credit Parties, (ii)
to make prepayments of Supplemental Rent, which prepayments shall be allocated
pro rata among all Loans and Holder Advances of all Properties and (iii) to
purchase one or more Properties at Termination Value to the extent no Default or
Event of Default shall have occurred or be continuing at such time and (b) to
change the percentage in the definition of "Maximum Residual Guarantee Amount"
to one hundred percent (100%);

         WHEREAS, the parties to this Amendment are in agreement with the option
in favor of the Lessee referenced in the preceding paragraph;

<PAGE>

         NOW, THEREFORE, IN CONSIDERATION of the premises and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties to this Amendment agree as follows:

                                   SECTION 1.
             DEFINITIONS; EFFECTIVE DATE; NO ADVERSE INTERPRETATION

         1.1 Capitalized terms used herein but not otherwise defined herein
shall have the meanings set forth in the Participation Agreement. The rules of
usage set forth in Appendix A to the Participation Agreement shall apply herein.
In the case of any conflict between the provisions of this Amendment and the
provisions of the Operative Agreements, the provisions of this Amendment shall
control construction of the terms; provided, however, that all of the terms and
provisions of this Amendment are in all respects subject to the provisions of
Section 12.16 of the Participation Agreement and the other usury savings
provisions set forth in the Operative Agreements.

         1.2 The amendments to the Operative Agreements as set forth in Section
2 of this Amendment shall only be effective from and after the date that all
conditions precedent set forth in Section 4 are satisfied or waived in writing
by the Majority Secured Parties; provided, however, that the Majority Secured
Parties shall not have the right or power to waive the condition precedent set
forth in Section 4(c)(iv) of this Amendment; and further provided, however, to
the extent all such conditions precedent are not so satisfied or waived on or
prior to December 31, 2003, then this Amendment shall be null and void and of no
further force or effect.

         1.3 The parties acknowledge and agree that the following amendments,
modifications and changes with respect to certain terms of the Operative
Agreements are implemented at the request of and as an accommodation to the
Credit Parties. Except as expressly set forth herein, the use of such
amendments, modifications and changes shall not be interpreted or construed in
any manner that would adversely affect the rights or remedies or would result in
any cost or expense to any party to any Operative Agreement other than a Credit
Party, or that would limit the responsibility of the Credit Parties under the
Operative Agreements, and the Credit Parties agree to indemnify and hold
harmless each such party from any such adverse effect, cost or expense.

                                   SECTION 2.
                       AMENDMENTS TO OPERATIVE AGREEMENTS

         2.1 Amendments to Participation Agreement.

             (a) Section 8.3(j) of the Participation Agreement is hereby amended
         and restated to read as follows:

                 "(j)     [Reserved]."

                                        2

<PAGE>

             (b) Section 8.7(b)(ii)(C) of the Participation Agreement is hereby
         amended by adding after the words "pursuant to Section 20.3 of the
         Lease", the following:

             "or the Equipment Release Value in connection with the exercise of
             the Equipment purchase option under Section 20.4 of the Lease,"

             (c) A new Section 8.7(b)(iii)(A) is hereby added immediately after
         Section 8.7(b)(iii) of the Participation Agreement to read as follows:

                 "(iii) (A) If on any date the Agent or the Lessor shall receive
             any amount in respect of Section 21.1 of the Lease, then such
             amount shall be applied to prepay the principal balance of the
             Loans and the Holder Advances, on a pro rata basis."

             (d) The first sentence in Section 8.8 of the Participation
         Agreement is hereby amended and restated to read as follows:

                 "If the Lessee shall at any time purchase any Property or any
             item(s) of Equipment pursuant to the Lease, or if the Properties
             shall be sold in accordance with Article XXII of the Lease, then,
             upon satisfaction by the Owner Trustee of its obligation to prepay
             the Loans and Holder Advances and upon payment of all other amounts
             owing to the Financing Parties under the Operative Agreements, the
             Agent is hereby authorized and directed to release such Properties
             and Equipment from the Liens created by the Security Documents to
             the extent of its interest therein."

             (e) Section 12.2 of the Participation Agreement is hereby amended
         to change the address for the Agent as follows:

                 "If to the Agent, to it at the following address:

                        Wachovia Bank, National Association
                        c/o Wachovia Securities, Inc.
                        301 South College Street
                        16th Floor
                        Charlotte, North Carolina 28288-0174
                        Attention:     Gabrielle Braverman
                        Telephone:     (704) 383-1967
                        Telecopy:      (704) 383-8108

                                        3

<PAGE>

                 with a copy to:

                        Wachovia Bank, National Association
                        301 South College Street
                        5th Floor
                        Charlotte, North Carolina 28288-0760
                        Attention:     Joyce Barry
                        Telephone:     (704) 374-4151
                        Telecopy:      (704) 374-4793"

             (f) The definition of "Maximum Residual Guarantee Amount" in
         Appendix A to the Participation Agreement is hereby amended and
         restated to read as follows:

             " "Maximum Residual Guarantee Amount" shall mean an amount equal to
             the product of the aggregate Property Cost for all Properties times
             one hundred percent (100%)."

             (g) The following definitions are added in appropriate alphabetical
         order in Appendix A to the Participation Agreement as follows:

             " "Equipment Release Date" shall have the meaning given to such
             term in Section 20.4 of the Lease."

             " "Equipment Release Option" shall have the meaning given to such
             term in Section 20.4 of the Lease."

             " "Equipment Release Value" shall have the meaning given to such
             term in Section 20.4 of the Lease."

             " "First Amendment Closing Date" shall mean the effective date of
             the First Amendment to Certain Operative Agreements pursuant to
             Section 4 thereof."

         2.2 Amendments to Lease.

             (a) The final sentence in Section 8.2(b) of the Lease is hereby
         amended and restated to read as follows:

                 "The Improvements respecting each particular Property shall be
             located only at the location identified in the applicable Lease
             Supplement. From time to time, Lessee shall have the right to move
             the Equipment to one or more other locations owned or leased by a
             Credit Party; provided, (i) at all times until the Financing
             Parties have been paid in full, Lessee shall cause each Lien in
             favor of the Agent pursuant to the Operative Agreements on each
             item of Equipment to be a perfected Lien of first priority under
             applicable law, subject only to Permitted Liens and Lessor Liens;
             (ii) on each June 30 and December 31 (or if either such day is not
             a Business Day, then on the next occurring Business Day) after the
             First Amendment

                                       4

<PAGE>

             Closing Date, Lessee shall certify (in form and substance
             reasonably satisfactory to Lessor) in writing to Lessor and the
             Agent (A) the location of each item of Equipment which has been
             moved to a location other than the location described in the Lease
             Supplement applicable to such item of Equipment and (B) that Lessee
             is in compliance with Section 8.2(b)(i); (iii) after the occurrence
             of any Event of Default, Lessee shall (if so requested in writing
             by Lessor) move, at Lessee's cost and expense, each item of
             Equipment to the Property at the location described in the Lease
             Supplement applicable to such item of Equipment; and (iv) Lessee
             shall not at any time locate, or permit to be located, any item of
             Equipment at any location which is, in whole or in part, leased to
             or occupied by any Person other than a Credit Party, except in
             connection with a lease or occupancy by any physician, physician
             practice group or healthcare entity with which any Credit Party is
             then transacting business in the ordinary course."

             (b) Section 10.1(b) of the Lease is hereby deleted.

             (c) The first sentence in Section 20.1 of the Lease is hereby
         amended and restated to read as follows:

             "Not less than one hundred eighty (180) days (or respecting the
             Purchase Option only, not less than sixty (60) days) and no more
             than two hundred forty (240) days prior to the Expiration Date or,
             respecting the Purchase Option and the Partial Purchase Option
             only, not less than thirty (30) days and no more than two hundred
             forty (240) days prior to any Payment Date (such Expiration Date
             or, respecting the Purchase Option and the Partial Purchase Option
             only, any such Payment Date being hereinafter referred to as the
             "Election Date"), Lessee may give Lessor irrevocable written notice
             (the "Election Notice") that Lessee is electing to exercise either
             (a) with respect to an Election Notice given in connection with any
             Payment Date or with respect to the Partial Purchase Option only
             any other date acceptable to the Agent prior to the Expiration
             Date), the option to purchase all, but not less than all, the
             Properties or the option to purchase certain individual Properties
             listed in Schedule A attached hereto (the option to purchase, prior
             to the Expiration Date, Properties listed in Schedule A, the
             "Partial Purchase Option"); provided, Lessee may not make the
             election under this Section 20.1(a) unless no Default or Event of
             Default shall have occurred and be continuing, either at the date
             any such election is made or at the applicable Election Date or
             other purchase date acceptable to the Agent prior to the Expiration
             Date (other than those that will be cured by the payment of the
             Termination Value for all applicable Properties), (b) with respect
             to an Election Notice given in connection with the Expiration Date
             only, the option to purchase all, but not less than all, the
             Properties on the Expiration Date (together with the option in
             subsection (a) to purchase all but not less than all of the
             Properties, each the "Purchase Option"), (c) with respect to an
             Election Notice given in connection with the Expiration Date only,
             the option to remarket all, but not less than all, the Properties
             to a Person other than Lessee or any Affiliate of Lessee and cause
             a sale of such Properties to occur on the Expiration Date pursuant
             to the terms of Section 22.1 (the "Sale Option") or (d) with
             respect to an Election Notice given in

                                        5

<PAGE>

             connection with the Expiration Date only, the option to renew the
             Term for all, but not less than all, the Properties for an
             additional period of one year pursuant to the terms of Section 2.2
             (the "Extension Option")."

             (d) A new Section 20.4 is hereby added immediately after Section
         20.3 of the Lease to read as follows:

                 "20.4 Equipment Purchase Option.

                 From time to time during the Term, upon at least thirty (30)
             days and no more than two hundred forty (240) days prior written,
             irrevocable notice to Lessor, Lessee shall have the right (the
             "Equipment Release Option") to purchase any items(s) of Equipment
             titled to Lessor specified in Lessee's written notice to Lessor on
             any Business Day prior to the Expiration Date set forth in such
             notice (the "Equipment Release Date"), provided, Lessee may not
             make this election and may not purchase the applicable Equipment
             unless (a) no Default or Event of Default shall have occurred and
             be continuing, either on the date the notice is given or on the
             Equipment Release Date (other than those that will be cured by the
             payment of the Equipment Release Value for the applicable
             Equipment) and no Default or Event of Default relating to such
             Equipment and/or the purchase thereof shall exist immediately after
             such purchase, (b) on or prior to the date the notice is given (i)
             Lessee shall have provided to Lessor an invoice respecting the
             applicable Equipment (such invoice to be in form and substance
             reasonably satisfactory to Lessor) from the seller of such
             Equipment documenting the sales price thereof, delivery and
             installation charges therefor, tax amounts paid therefor and any
             and all other amounts (both hard costs and soft costs) paid to or
             for the benefit of such seller respecting such Equipment or (ii)
             Lessee shall have provided to Lessor other documentation supporting
             the subject Equipment Release Value (such documentation to be in
             form and substance reasonably satisfactory to Lessor) to the extent
             such an invoice as described in the above subsection (b)(i) is not
             available and (c) Lessee shall have provided to Lessor on or prior
             to the date the notice is given an officer's certificate from a
             Responsible Officer respecting the applicable Equipment (such
             officer's certificate to be in form and substance reasonably
             satisfactory to Lessor) specifying all amounts described in the
             preceding subsection (b) and all other amounts funded under the
             Operative Agreements by any of the Financing Parties respecting
             such Equipment. (Collectively, respecting the applicable Equipment,
             the amounts described in such officer's certificate (including
             without limitation the amounts described in the foregoing
             subsections (b) and (c)) shall be referred to as the "Equipment
             Release Value".)

                 With respect to an election by Lessee to exercise its Equipment
             Release Option, Lessee shall pay to Lessor on the Equipment Release
             Date the Equipment Release Value attributable to such Equipment. On
             the Equipment Release Date, Lessee shall also pay (but without
             duplication of such payment of Equipment Release Value) to Lessor
             and all other parties, as appropriate, the sum of all costs and
             expenses incurred by Lessor and/or the Agent (as the case may be)
             in

                                        6

<PAGE>

             connection with such purchase of Equipment, all Rent and all other
             amounts then due and payable under this Lease and/or any other
             Operative Agreements and solely with respect to the Equipment then
             being purchased, all Rent and all other amounts then accrued under
             this Lease and/or any other Operative Agreements (and upon payment
             thereof, such amounts shall be satisfied for purposes of all
             Operative Agreements).

                 Upon Lessee's payment to Lessor of the Equipment Release Value
             for the applicable Equipment and to the appropriate parties of the
             other amounts referenced in the prior paragraph, Lessor shall
             execute, acknowledge and deliver to Lessee, at Lessee's cost and
             expense each of the following: (a) a release of the applicable
             Equipment from the Lease and (b) a Bill of Sale conveying the
             Equipment to Lessee (or to any entity designated by Lessee) free
             and clear of the Lien of this Lease, the Lien of the Credit
             Documents, and any Lessor Liens and on an "AS IS, WHERE IS" basis
             in its then present physical condition."

             (e) Section 21.1 of the Lease is hereby amended and restated to
         read as follows:

                 "21.1 On each Payment Date, Lessee may elect to pay
             Supplemental Rent for the purpose of application of such payment to
             the then current Property Cost of each Property on a ratable basis
             in accordance with Section 8.7(b)(iii)(A) of the Participation
             Agreement. Prior to making any such payment, Lessee shall give
             Lessor no less than three (3) Business Days notice (such notice to
             be in form and substance reasonably satisfactory to Lessor) of such
             payment and the amount thereof. Each such payment shall reduce the
             Property Cost of each Property on a ratable basis based on a
             comparison of the then current individual Property Cost of each
             Property to the then current aggregate Property Cost of all
             Properties."

             (f) Section 25.2(b) of the Lease is hereby amended and restated to
         read as follows:

                 "(b) Without the prior written consent of the Agent, any
             Lender, any Holder or Lessor, but subject to the other provisions
             of this Section 25.2, Lessee may sublet any Property or portion
             thereof (i) to any Affiliate of any Credit Party and/or to any
             other physician, physician practice group or healthcare entity with
             which any Credit Party (A) is then transacting business in the
             ordinary course, (B) is then intending to transact business in the
             ordinary course and/or (C) previously transacted business in the
             ordinary course and (ii) to any physician, physician practice group
             or other healthcare entity transacting an oncology business in the
             ordinary course (which does not satisfy the criteria of the
             foregoing Section 25.2(b)(i)). Notwithstanding the foregoing, the
             aggregate Property Cost of Properties subject to subleasing as
             described in the foregoing Sections 25.2(b)(i)(C) and 25.2(b)(ii)
             shall not any time exceed fifty percent (50%) of the then current
             outstanding Property Cost; provided, however, no Equipment shall be
             permitted to be subleased unless such sublease is also with respect
             to the related real estate of the subject Property or portion
             thereof. Except as referenced in this subsection (b), no

                                        7

<PAGE>

             other subleases shall be permitted unless consented to in writing
             by Lessor. All subleasing shall be done on market terms and shall
             in no way diminish the fair market value or useful life of any
             applicable Property."

             (g) Schedule A of the Lease is hereby amended and restated to read
         as follows:

                                   "Schedule A
             Any and all Properties, whether individually or in groups"

         2.3 Amendments to Trust Agreement.

             (a) Section 3.4(b) of the Trust Agreement is hereby amended by
         adding after the words "pursuant to Section 20.3 of the Lease," the
         following:

             "or (iv) the exercise of the right of the Lessee to make one or
             more partial prepayments respecting the Properties pursuant to
             Section 21.1 of the Lease,"

             (b) Schedule I of the Trust Agreement is hereby amended to change
         the address for Wachovia Bank, National Association (as successor in
         interest to First Union National Bank) as follows:

                 "Wachovia Bank, National Association
                 c/o Wachovia Securities, Inc.
                 301 South College Street
                 16th Floor
                 Charlotte, North Carolina 28288-0174
                 Attention:     Gabrielle Braverman
                 Telephone:     (704) 383-1967
                 Telecopy:      (704) 383-8108

                 with a copy to:

                 Wachovia Bank, National Association
                 301 South College Street
                 5th Floor
                 Charlotte, North Carolina 28288-0760
                 Attention:     Joyce Barry
                 Telephone:     (704) 374-4151
                 Telecopy:      (704) 374-4793"

         2.4 Amendments to Credit Agreement.

             (a) Section 2.6(b) of the Credit Agreement is hereby amended by
         adding after the words "pursuant to Section 20.3 of the Lease," the
         following:

                                        8

<PAGE>

             "or (iv) the exercise of the right of the Lessee to make one or
             more partial prepayments respecting the Properties pursuant to
             Section 21.1 of the Lease,"

             (b) Schedule 1.1 of the Credit Agreement, which appears immediately

         after the signature pages for the Credit Agreement and immediately
         before Exhibit A-1, is hereby amended (i) to add "Schedule 1.1" to the
         top of such schedule and (ii) to change the address for Wachovia Bank,
         National Association (as successor in interest to First Union National
         Bank) as follows:

                 "Wachovia Bank, National Association
                 c/o Wachovia Securities, Inc.
                 301 South College Street
                 16th Floor
                 Charlotte, North Carolina 28288-0174
                 Attention:     Gabrielle Braverman
                 Telephone:     (704) 383-1967
                 Telecopy:      (704) 383-8108

                 with a copy to:

                 Wachovia Bank, National Association
                 301 South College Street
                 5th Floor
                 Charlotte, North Carolina 28288-0760
                 Attention:     Joyce Barry
                 Telephone:     (704) 374-4151
                 Telecopy:      (704) 374-4793"

                                   SECTION 3.
                         REPRESENTATIONS AND WARRANTIES

         In order to induce the parties to this Amendment (other than the Credit
Parties) to execute and deliver this Amendment, each Credit Party hereby
represents and warrants to all other parties to this Amendment that, as of the
date hereof, (a) the representations and warranties of such Credit Party set
forth in Section 6.2 of the Participation Agreement are true and correct (except
for any such representations and warranties which relate solely and expressly to
an earlier time) and (b) after giving effect to this Amendment, no Default or
Event of Default has occurred and is continuing or shall result from this
Amendment.

                                   SECTION 4.
                              CONDITIONS PRECEDENT

         The amendments to the Operative Agreements set forth in Section 2 of
this Amendment shall be effective from and after the date of satisfaction or
waiver by the Majority Secured Parties

                                        9

<PAGE>

of the following conditions precedent on or prior to, but not after, December
31, 2003; provided, however, that the Majority Secured Parties shall not have
the right or power to waive the condition precedent set forth in Section
4(c)(iv) of this Amendment; and further provided, however, to the extent such
conditions precedent have not been satisfied or waived as referenced in the
preceding clause on or prior to, but not after, December 31, 2003, this
Amendment shall be null and void and of no further force or effect:

             (a) Executed Counterparts. Receipt by the Agent of executed
         counterparts of this Amendment by the parties hereto and such other
         documents, agreements or instruments, other than legal opinions, deemed
         necessary or advisable by the Agent (each in form and substance
         reasonably satisfactory to the Agent).

             (b) Corporate Authorization. Receipt by the Agent of a certificate
         of the Secretary or an Assistant Secretary of each Credit Party (in
         substantially the form of Exhibit A-1, Exhibit A-2 or Exhibit A-3, as
         applicable, attached hereto) certifying as to (i) attached copies of
         resolutions adopted by the Board of Directors (or comparable governing
         body or entity) of such Credit Party approving and authorizing the
         execution, delivery and performance of this Amendment and (ii) the
         incumbency of the officer or officers of such Credit Party executing
         this Amendment and the certificates, instruments and other documents
         related hereto.

             (c) Officer's Certificate. Receipt by the Agent of a certificate of
         a Responsible Officer of the Lessee (in substantially the form of
         Exhibit B attached hereto) certifying that (i) after giving effect to
         this Amendment, no Default or Event of Default shall have occurred and
         be continuing or shall have resulted from the consummation of the
         transactions contemplated by this Amendment, (ii) all representations
         and warranties of each Credit Party set forth in any Operative
         Agreement are true and correct (except for any such representations and
         warranties which relate solely and expressly to an earlier time), (iii)
         each Operative Agreement to which any Credit Party is a party is in
         full force and effect with respect to it and (iv) the Lessee thereby
         requests that this Amendment be confirmed to be in full force and
         effect as of a date to be specified in such certificate, which in any
         event shall be no later than December 31, 2003.

             (d) Opinion of Counsel. Receipt by the Agent of one or more legal
         opinions of counsel to each Credit Party relating to this Amendment in
         form and substance reasonably satisfactory to the Agent.

             (e) Fees and Expenses. Receipt by (i) the Agent of an amendment fee
         equal to the product of (A) the sum of the Holder Commitments of the
         Holders approving this Amendment plus the Lender Commitments of the
         Lenders approving this Amendment, multiplied by (B) 12.5 basis points
         (0.125%) (such amendment fee to be distributed by the Agent to such
         approving Lenders and such approving Holders ratably based on their
         outstanding Loans and Holder Advances) and (ii) the Agent or the
         Agent's designee of the out-of-pocket expenses of the Agent in
         connection with the negotiation, preparation, execution and delivery of
         this Amendment and the other transactions contemplated herein,
         including without limitation reasonable legal fees and expenses, such
         fees and expenses in

                                       10

<PAGE>

         the foregoing clauses (i) and (ii) being subject in all respects to the
         provisions of Section 12.16 of the Participation Agreement and the
         other usury savings provisions set forth in the Operative Agreements.

             (f) No Default or Event of Default. The Agent shall not have
         received any notice from any Credit Party or from any Financing Party
         that any Default or Event of Default shall have occurred and be
         continuing as of the date of satisfaction or waiver of the conditions
         precedent set forth in this Section 4.

             (g) Agent's Certificate. The Agent shall deliver a certificate to
         the Lessee, each Lender and each Holder in substantially the form of
         Exhibit C attached hereto to the extent, but only to the extent, that
         (i) one or more of the conditions precedent set forth in this Section 4
         shall have been waived in writing by the Majority Secured Parties and
         the other such conditions precedent shall have been satisfied or (ii)
         (A) the Agent shall have received the documentation specified in
         Sections 4(a)-4(d), (B) the Agent or the Agent's designee shall have
         received the fees, expenses and other amounts specified in Section 4(e)
         and (C) the Agent shall not have received any notice specified in
         Section 4(f). The Agent shall have the obligation to deliver such
         certificate on a timely basis upon the satisfaction and/or waiver of
         the conditions set forth in Sections 4(a)-4(f) of this Amendment.

                                   SECTION 5.
                                  MISCELLANEOUS

         5.1 Each of the Lenders and Holders which are parties hereto hereby
instructs the Agent and the Lessor to enter into this Amendment.

         5.2 Except as modified hereby, all of the terms and provisions of the
Operative Agreements (including Schedules and Exhibits) shall remain in full
force and effect, and each Credit Party and Financing Party hereby ratifies and
affirms the terms and conditions of the Operative Agreements as modified hereby.

         5.3 This Amendment may be executed in any number of counterparts, each
of which when so executed and delivered shall be deemed an original and it shall
not be necessary in making proof of this Amendment to produce or account for
more than one such counterpart.

         5.4 Without regard as to whether this Amendment is ever made effective
or whether any of the conditions precedent set forth in Section 4 are satisfied
or waived by the Majority Secured Parties, the Credit Parties shall cause (a)
the fees described in Section 4(e)(i) to be paid concurrently with the execution
and delivery of this Amendment by the Credit Parties and the Majority Secured
Parties and (b) the fees and expenses described in Section 4(e)(ii) to be
promptly paid (and in any event within thirty (30) days) after submission of an
invoice or other written request for payment of the same.

         5.5 THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES
HEREUNDER SHALL BE GOVERNED BY AND CONSTRUED, INTERPRETED

                                       11

<PAGE>

AND ENFORCED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NORTH CAROLINA EXCEPT
AS TO THE TRUST AGREEMENT WHICH SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE
WITH THE LAWS OF THE STATE OF UTAH.

         5.6 THE PROVISIONS OF THE PARTICIPATION AGREEMENT RELATING TO
SUBMISSION TO JURISDICTION, VENUE AND ARBITRATION ARE HEREBY INCORPORATED BY
REFERENCE HEREIN, MUTATIS MUTANDIS.

                  [Remainder of Page Intentionally Left Blank]

                                       12

<PAGE>

         IN WITNESS WHEREOF, each of the parties hereto has caused a counterpart
of this Amendment to be duly executed and delivered as of the date first above
written.

                              AOR SYNTHETIC REAL ESTATE, INC., as the Lessee and
                                      as a Tranche A Guarantor

                              AOR REAL ESTATE, INC., as a Tranche A Guarantor
                              US ONCOLOGY CORPORATE, INC., as a Tranche A
                                      Guarantor
                              RMCC CANCER CENTER, INC., as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF OREGON, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF INDIANA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF MISSOURI, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF ARIZONA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF SOUTH CAROLINA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF OKLAHOMA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF PENNSYLVANIA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF VIRGINIA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF NORTH CAROLINA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF NEW YORK, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF FLORIDA, INC.
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF NEVADA, INC.
                                      as a Tranche A Guarantor
                              AOR HOLDING COMPANY OF INDIANA, INC.,
                                      as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF TEXAS, INC.,
                                      as a Tranche A Guarantor
                              AORT HOLDING COMPANY, INC., as a Tranche A
                                      Guarantor
                              AORIP, INC., as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF ALABAMA, INC.,
                                      as a Tranche A Guarantor
                              PHYSICIAN RELIANCE NETWORK, INC., as a Tranche A
                                      Guarantor
                              US ONCOLOGY RESEARCH, INC., as a Tranche A
                                      Guarantor
                              ALABAMA PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor
                              CALIFORNIA PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor
                              MICHIGAN PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor
                              IOWA PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor
                              NEBRASKA PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor
                              ST. LOUIS PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor
                              NEW MEXICO PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor
                              WASHINGTON PHARMACEUTICAL SERVICES, LLC,
                                      as a Tranche A Guarantor

                           [Signature pages continued]

<PAGE>

                              TOPS PHARMACY SERVICES, INC., as a Tranche A
                                      Guarantor
                              AOR MANAGEMENT COMPANY OF CENTRAL FLORIDA,
                                      INC., as a Tranche A Guarantor
                              AOR MANAGEMENT COMPANY OF OHIO, INC., as a
                                      Tranche A Guarantor
                              GREENVILLE RADIATION CARE, INC., as a Tranche A
                                      Guarantor
                              PHYSICIAN RELIANCE HOLDINGS, LLC, as a Tranche A
                                      Guarantor
                              PRN PHYSICIAN RELIANCE, LLC, as a Tranche A
                                      Guarantor

                              By: _______________________________________
                              Name:  Bruce D. Broussard
                              Title: Vice President of each of the foregoing
                                     Entities


                              AOR of Texas Management Limited Partnership,
                              a Texas limited partnership, as a Tranche A
                              Guarantor

                              By:     AOR Management Company of Texas, Inc.,
                                      a Delaware corporation, as general partner

                                      By: ________________________________
                                      Name:  Bruce D. Broussard
                                      Title: Vice President

                              AOR of Indiana Management Partnership,
                              an Indiana general partnership, as a Tranche A
                              Guarantor

                              By:     AOR Management Company of INDIANA, Inc., a
                                      Delaware corporation, as general partner

                                      By: _____________________________
                                      Name:  Bruce D. Broussard
                                      Title: Vice President


                              By:     AOR HOLDING Company of INDIANA, Inc., a
                                      Delaware corporation, as general partner

                                      By: _________________________________
                                      Name:  Bruce D. Broussard
                                      Title: Vice President

                           [Signature pages continued]

<PAGE>

                              PHYSICIAN RELIANCE, LP, a Texas limited
                              partnership, as a Tranche A Guarantor

                              By:     PRN PHYSICIAN RELIANCE, LLC, a Texas
                                      limited liability company, as general
                                      partner

                                      By: _________________________________
                                      Name:  Bruce D. Broussard
                                      Title: Vice President


                              US ONCOLOGY, INC., as the Guarantor and as a
                              Tranche A Guarantor

                              By: ______________________________
                              Name:  Bruce D. Broussard
                              Title: Chief Financial Officer

                           [Signature pages continued]

<PAGE>

                              WELLS FARGO BANK NORTHWEST, NATIONAL
                              ASSOCIATION, not individually, but solely as
                              Owner Trustee under the AOR Trust 1997-1

                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________

                           [Signature pages continued]

<PAGE>

                              WACHOVIA BANK, NATIONAL ASSOCIATION (as
                              successor in interest to First Union
                              National Bank), as Agent, as a Lender, and
                              as a Holder


                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________

                           [Signature pages continued]

<PAGE>

                              UBS AG, STAMFORD BRANCH, as a Lender and as a
                              Holder


                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________


                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________

                           [Signature pages continued]

<PAGE>

                              GENERAL ELECTRIC CAPITAL CORPORATION, as a Lender
                              and as a Holder


                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________

                           [Signature pages continued]

<PAGE>

                              DEUTSCHE BANK TRUST COMPANY AMERICAS (formerly
                              known as Bankers Trust Company), as a Lender and
                              as a Holder


                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________

                           [Signature pages continued]

<PAGE>

                              JPMORGAN CHASE BANK, as a Lender and as a Holder


                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________

                           [Signature pages continued]

<PAGE>

                              CREDIT LYONNAIS NEW YORK BRANCH, as a Lender


                              By: ___________________________________
                              Name: _________________________________
                              Title: ________________________________


                              [Signature pages end]