<PAGE>

                                                                    EXHIBIT 10.2

                           THIRD AMENDMENT TO CERTAIN
                              OPERATIVE AGREEMENTS


     THIS THIRD AMENDMENT TO CERTAIN OPERATIVE AGREEMENTS dated as of May 14,
1999 (this "Amendment") is by and among AOR SYNTHETIC REAL ESTATE, INC., a
Delaware corporation (the "Lessee" or the "Construction Agent" or a "Tranche A
Guarantor"); AMERICAN ONCOLOGY RESOURCES, INC., a Delaware corporation (the
"Guarantor" or a "Tranche A Guarantor"); the various entities which are parties
hereto as guarantors of the Tranche A Loans (subject to the definition of
Tranche A Guarantors in Appendix A of the Participation Agreement, as defined
below, individually a "Tranche A Guarantor" and collectively, the "Tranche A
Guarantors"); 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 hereto as lenders
(subject to the definition of Lenders in Appendix A of the Participation
Agreement, individually, a "Lender" and collectively, the "Lenders"); the
various banks and other lending institutions which are parties hereto as holders
of certificates issued with respect to the AOR Trust 1997-1 (subject to the
definition of Holders in Appendix A of the Participation Agreement,
individually, a "Holder" and collectively, the "Holders"); and FIRST UNION
NATIONAL BANK, a national banking association ("FUNB"), 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").


                              W I T N E S S E T H:

     WHEREAS, pursuant to a certain Assignment and Assumption Agreement dated on
or about the date hereof (as amended, modified, supplemented, restated and/or
replaced from time to time, the "Loan Assignment to First Union") among the
various banks and other lending institutions which were parties to the Existing
Participation Agreement, in their capacity as lenders (in such capacity, the
"Prior Lenders"), FUNB and the other parties referenced on the signature pages
thereto, the Prior Lenders assigned to FUNB, and FUNB assumed, the right, title
and interest of the Prior Lenders, in their capacity as lenders (subject to
limited exceptions and all as more fully set forth therein);

     WHEREAS, pursuant to a certain Assignment and Assumption Agreement dated on
or about the date hereof (as amended, modified, supplemented, restated and/or
replaced from time to time, the "Holder Advance Assignment to First Union")
among the various banks and other lending institutions which were parties to the
Existing Participation Agreement, in their capacity as holders (in such
capacity, the "Prior Holders"), FUNB and the other parties referenced on the
signature pages thereto, the Prior Holders assigned to FUNB, and FUNB assumed,
the right, title and interest of the Prior Holders, in their capacity as holders
(subject to limited exceptions and all as more fully set forth therein);
<PAGE>

     WHEREAS, pursuant to a certain Assignment and Assumption Agreement dated on
or about the date hereof (as amended, modified, supplemented, restated and/or
replaced from time to time, the "Loan Assignment to the New Lenders") among
FUNB, the Lenders and the other parties referenced on the signature pages
thereto, FUNB assigned to the Lenders, and the Lenders assumed, the right, title
and interest of FUNB, in its capacity as lender (subject to limited exceptions
and all as more fully set forth therein);

     WHEREAS, pursuant to a certain Assignment and Assumption Agreement dated on
or about the date hereof (as amended, modified, supplemented, restated and/or
replaced from time to time, the "Holder Advance Assignment to the New Holders")
among FUNB, the Holders and the other parties referenced on the signature pages
thereto, FUNB assigned to the Holders, and the Holders assumed, the right, title
and interest of FUNB, in its capacity as holder (subject to limited exceptions
and all as more fully set forth therein);

     WHEREAS, prior to the effective date of the Loan Assignment to First Union,
the Holder Advance Assignment to First Union, the Loan Assignment to the New
Lenders and the Holder Advance Assignment to the New Holders (collectively, the
"Assignments"), the Prior Lenders, the Prior Holders, the Lessee, the Guarantor,
the Tranche A Guarantors, the Owner Trustee and FUNB, as the agent for the Prior
Lenders and respecting the Security Documents, as the agent for the Prior
Lenders and the Prior Holders, were parties to the Participation Agreement dated
as of December 30, 1997, as amended by the First Amendment to Participation
Agreement and Credit Agreement dated as of January 30, 1998 and as further
amended by the Second Amendment to certain Operative Agreements dated as of
November 30, 1998 (as so amended, and as further amended, modified,
supplemented, restated and/or replaced from time to time prior to the date of
this Agreement, the "Existing Participation Agreement");

     WHEREAS, the Existing Participation Agreement, as amended by this
Amendment, may be referred to herein as the "Participation Agreement";

     WHEREAS, the parties to this Amendment wish to amend certain agreements,
instruments and other documents to which they are a party (or to which certain
of them are a party) in connection with a lease financing arrangement provided
in favor of Lessee by the other parties to this Amendment with regard to the
Properties; and

     WHEREAS, after the effective date of the above-referenced assignments and
after the effective date of this Amendment, the parties to this Amendment intend
for the Loans, the Holder Advances and the other obligations evidenced by the
Operative Agreements to constitute the same Loans, Holder Advances and other
such obligations that existed prior to the effective date of such assignments
and prior to the effective date of this Amendment and not to constitute a
novation.

                                       2
<PAGE>

                               A G R E E M E N T

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

     1.  Definitions.  Capitalized terms used herein but not otherwise defined
herein shall have the meaning set forth therefor in Appendix A to the
Participation Agreement.  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.

     2.  Agency Agreement.  The Agency Agreement is amended as follows:

          (a) Section 3.2(a) is amended by adding the following sentence as the
     final sentence thereto:

          "Notwithstanding the foregoing, it is specifically understood and
          agreed that if at any time the total Property Costs remaining to be
          expended exceed the Unfunded Amount, the Construction Agent shall have
          the right to purchase or cause the Lessee to purchase any one or more
          of the Properties as set forth in Section 5.3(c) and the limitations
          on recourse set forth in Section 5.4 shall not apply.";

          (b) Section 3.3 is amended by deleting it in its entirety and
     replacing it with the following:

     "3.3  Failure to Complete Construction Period Properties and Purchase
Obligation.

               If at any time prior to the Completion Date with respect to any
          Construction Period Property (a) there occurs a Casualty, or
          Condemnation, the loss from which exceeds or is expected to exceed ten
          percent (10%) of the aggregate Construction Budget for such
          Construction Period Property or that will prevent such Construction
          Period Property from being completed by the Construction Period
          Termination Date, (b) there shall occur any Environmental Violation
          which the Lessor deems material in its reasonable judgment, (c) there
          shall occur a Force Majeure Event which lasts beyond three (3) months,
          (d) the Construction Agent shall abandon or permanently discontinue
          the construction and development of such Construction Period Property
          (which abandonment or permanent discontinuance shall be deemed to have
          occurred if no work at such Construction Period Property site is
          undertaken or completed during a period of thirty (30) days or more
          for reasons other than a Force Majeure Event) and such abandonment or
          permanent discontinuance shall be determined by an arbitrator in
          accordance with the provisions of Section 12.7 of the Participation
          Agreement to be an act of willful misconduct, fraud or an illegal act
          on the part of the Construction Agent or (e) the Construction Agent
          shall abandon or permanently discontinue the construction and
          development of such Construction Period

                                       3
<PAGE>

          Property (which abandonment or permanent discontinuance shall be
          deemed to have occurred if no work at such Construction Period
          Property site is undertaken or completed during a period of thirty
          (30) days or more for reasons other than a Force Majeure Event) and
          either no arbitration proceeding referred to in the foregoing
          subsection (d) shall have been commenced within sixty (60) days from
          the date each of the parties hereto has knowledge of such abandonment
          or permanent discontinuance or an arbitrator, in a proceeding in
          accordance with the provisions of Section 12.7 of the Participation
          Agreement, shall determine such abandonment or permanent
          discontinuance is not an act of willful misconduct, fraud or an
          illegal act on the part of the Construction Agent then on a date
          (subject to the following two sentences) designated by the Lessor (v)
          in the case of the foregoing subsections (b) and (d), the Construction
          Agent shall pay to Lessor (and the limitations on recourse set forth
          in Section 5.4 shall not apply) an aggregate amount equal to the
          liquidated damages amount referenced in Section 5.3(b) of this
          Agreement regarding such Construction Period Property and on such
          date, so long as such liquidated damages amount is paid in full,
          Lessor shall transfer and convey to the Construction Agent all right,
          title and interest of Lessor in and to such Construction Period
          Property and all insurance, condemnation and other proceeds with
          respect to such Construction Period Property (to the extent such
          proceeds do not constitute a portion of such liquidated damages
          amount) and (w) in the case of the foregoing subsections (a), (c) and
          (e), the Construction Agent shall have the option (i) to pay to Lessor
          an aggregate amount equal to the liquidated damages amount referenced
          in Section 5.3(b) of this Agreement regarding such Construction Period
          Property (and the limitations on recourse set forth in Section 5.4
          shall not apply) and on such date Lessor shall transfer and convey to
          the Construction Agent all right, title and interest of Lessor in and
          to such Construction Period Property and all insurance, condemnation
          and other proceeds with respect to such Construction Period Property
          (to the extent such proceeds do not constitute a portion of such
          liquidated damages amount) or (ii) (subject to the limitations of all
          applicable usury laws and the usury savings provisions of the
          Operative Agreements, including, without limitation, Section 12.16 of
          the Participation Agreement and Section 30.14 of the Lease) to pay to
          Lessor the Maximum Amount regarding such Construction Period Property
          and abandon all right, title and interest of the Construction Agent in
          and to such Construction Period Property. If either the foregoing
          subsections (d) or (e) applies to any Construction Period Property,
          then the Construction Agent shall pay the Lessor on a date designated
          by the Lessor the Maximum Amount as aforesaid, but without regard as
          to whether any applicable arbitration proceedings (if any) have been
          completed. To the extent subsection (d) is thereafter determined by
          such arbitrator to apply, then such Maximum Amount previously paid
          will be credited against the liquidated damages amount referenced in
          Section 5.3(b) of this Agreement. Upon payment of the Maximum Amount
          (except during the pendency of any arbitration proceeding referred to
          above or the sixty (60) day period referenced in the foregoing
          subsection (e)) respecting any particular Construction Period Property
          (as calculated for such Construction

                                       4
<PAGE>

          Period Property only) as aforesaid, no further amounts shall be owed
          by any Credit Party with respect to such Construction Period Property,
          and Lessor and each person or entity acting by or through the Lessor,
          including without limitation the Agent, the Lenders and the Holders
          shall have no (i) right against any Credit Party to any additional
          amount regarding such Construction Period Property, or (ii) claim
          respecting any other Property to satisfy any amount owed by any Credit
          Party with respect to such Construction Period Property or that would
          otherwise have been owed by any Credit Party with respect to such
          Construction Period Property. The parties to the Third Amendment
          (including their successors and assigns) nonetheless acknowledge and
          agree that (x) even though the maximum aggregate recovery from the
          Credit Parties is limited by the Maximum Amount in the foregoing
          subsection (w)(ii), the Lessor's right of recovery from such
          Construction Period Property (as opposed to any recovery from the
          Credit Parties) is not so limited (subject to the limitations of all
          applicable usury laws and the usury savings provisions of the
          Operative Agreements, including, without limitation, Section 12.16 of
          the Participation Agreement and Section 30.14 of the Lease) and (y)
          the provisions of this Section 3.3 shall in no way limit or otherwise
          affect the obligations of the Lessee and the Construction Agent (or
          the recourse of the Agent, the Lenders and the Holders against the
          Lessee and the Construction Agent) with respect to the Properties that
          are not Construction Period Properties (whether such obligations or
          recourse arise under the Operative Agreements or otherwise). At the
          cost and expense of the Construction Agent with respect to any
          Construction Period Property to be conveyed to the Construction Agent
          pursuant to this Section 3.3, the Lessor shall convey such
          Construction Period Property for which an amount equal to the
          liquidated damages amount referenced in Section 5.3(b) has been paid
          "AS-IS, WHERE-IS" and in its then present physical condition to the
          Construction Agent or its designee free and clear of Lessor Liens. If
          the foregoing subsections (v) and/or (w) are not applicable to a
          particular Construction Period Property because the requirements of
          the foregoing subsections (a), (b), (c), (d) or (e) are not satisfied,
          then the Construction Agent shall promptly and diligently complete the
          development, acquisition, refinancing, installation, construction and
          testing of such Construction Period Property in accordance with the
          Plans and Specifications and with the terms hereof and cause the
          Completion Date with respect to such Construction Period Property to
          occur on or prior to the Construction Period Termination Date. Any
          determination that an Environmental Violation is immaterial for
          purposes of this Agreement shall not limit the obligations of Lessee
          respecting such Environmental Violation under the Lease. The
          Construction Agent shall notify the Lessor of the occurrence of any of
          the matters described in the foregoing subsections (a), (c), (d) or
          (e) promptly upon the Construction Agent gaining knowledge thereof and
          the Construction Agent shall perform its obligations under Section
          15.2 of the Lease regarding Environmental Violations as to any
          Property subject thereto."; and

          (c) A new Section 5.4 is added as follows:

                                       5
<PAGE>

          "5.4  LIMITATION ON RECOURSE.

               Notwithstanding anything contained herein or in any other
          Operative Agreement to the contrary (except as expressly stated in
          Sections 3.2(a) and 3.3 of this Agreement and in the usury savings
          provisions of the Operative Agreements, including, without limitation,
          Section 12.16 of the Participation Agreement and Section 30.14 of the
          Lease), upon the occurrence and during the continuance of an Agency
          Agreement Event of Default relating solely to one or more of the
          Construction Period Properties (other than as a result of (x) a Lease
          Event of Default arising under Sections 17.1(g), (h), (i) or (j) of
          the Lease or (y) any indemnity claim pursuant to the Operative
          Agreements including without limitation pursuant to Sections 11.1
          through 11.5 of the Participation Agreement, in either such case
          pursuant to the foregoing subsections (x) or (y), the limitations
          contained in this Section 5.4 shall not apply), the maximum aggregate
          amount that the Lessor, or any person or entity acting by or through
          the Lessor, including without limitation the Agent, the Lenders and
          the Holders, shall be entitled to recover from the Credit Parties on
          account of such Agency Agreement Event of Default with respect to each
          such Construction Period Property or Construction Period Properties
          shall be an amount equal to the sum of (collectively, the amounts
          referenced in the following provisions of this sentence may be
          referred to as the "Maximum Amount") (i) 89.9% of the aggregate
          Property Cost for all Construction Period Properties (unless Lessor
          upon the written advice of the Agent elects to exercise such remedy
          against fewer than all Construction Period Properties, in which case
          the amount for the foregoing portion of this subsection (i) shall be
          89.9% of the aggregate Property Cost for such Construction Period
          Properties), exclusive of the portion of the aggregate Property Cost
          expended for the purchase of the Land related to such Construction
          Period Properties (the "Land Cost") plus (ii) 100% of the Land Cost,
          plus (iii) all amounts owed by the Lessee or the Construction Agent
          under or with respect to any Operative Agreement in connection with
          any Environmental Violation (including without limitation
          Environmental Violations referred to in Section 3.3) related to such
          Construction Period Properties plus (iv) any loss, cost or damage
          suffered by the Lessor, the Agent or any Lender or Holder in
          connection with or as a result of (1) fraud, misapplication of funds,
          illegal acts or willful misconduct on the part of the Lessee or the
          Construction Agent or (2) any claim by any third party caused by or
          resulting from the Lessee's or the Construction Agent's actions or
          failure to act while in possession or control of any such Construction
          Period Property (it being understood and agreed that the Construction
          Agent shall be deemed to be in possession and control of each
          Construction Period Property at all times until such possession and
          control is relinquished pursuant to the terms of the Operative
          Agreements) minus (v) any amount relating to such Construction Period
          Properties expended by the Construction Agent on behalf of the Lessor
          if the Lessor is obligated to reimburse the Construction Agent for
          such amount but such reimbursement has not yet occurred. The parties
          to the Third Amendment (including their successors and assigns)
          nonetheless acknowledge and agree that

                                       6
<PAGE>

          (x) even though the maximum aggregate recovery from the Credit Parties
          is limited as aforesaid, the Lessor's right of recovery from the
          Construction Period Properties (as opposed to any recovery from the
          Credit Parties) is not so limited (subject to the limitations of all
          applicable usury laws and the usury savings provisions of the
          Operative Agreements, including, without limitation, Section 12.16 of
          the Participation Agreement and Section 30.14 of the Lease), (y) the
          provisions of this Section 5.4 shall in no way limit or otherwise
          affect the obligations of the Lessee and the Construction Agent (or
          the recourse of the Agent, the Lenders and the Holders against the
          Lessee and the Construction Agent) with respect to the Properties that
          are not Construction Period Properties (whether such obligations or
          recourse arise under the Operative Agreements or otherwise) and (z)
          the provisions of this Section 5.4 shall in no way limit or otherwise
          affect the right of the Construction Agent under Section 5.3(c) of
          this Agreement to purchase or cause the Lessee to purchase a Property
          to cure an Agency Agreement Event of Default regarding such Property
          (to the extent such Agency Agreement Event of Default is no longer
          continuing with respect to any other Property remaining subject to
          this Agreement after such purchase) for an amount equal to the
          liquidated damages amount set forth in Section 5.3(b) of this
          Agreement."

          (d) The following is added as subsection 4.1(c) to the Agency
     Agreement:

          "The Construction Agent shall have the option to submit as a Property
          Cost a requisition for the insurance premiums for insurance required
          by the Operative Agreements, including without limitation Article XIV
          of the Lease, to the extent such requisition is otherwise permitted
          and eligible for funding under the provisions of the Operative
          Agreements."

     3.   Participation Agreement.  The Participation Agreement is amended as
follows:

          (a) Section 2 of the Participation Agreement is amended by deleting
     each reference to "three percent (3%)" and replacing each with a reference
     to "three and four tenths percent (3.4%)";

          (b) Section 5.2(c) of the Participation Agreement is amended by:

               (i) deleting the reference to "ninety-seven percent (97%)" and
          replacing it with a reference to "ninety-six and six tenths percent
          (96.6%)";

               (ii) deleting the reference to "twelve percent (12%)" and
          replacing it with a reference to "eleven and six tenths percent
          (11.6%)"; and

               (iii)  deleting the reference to "three percent (3%)" and
          replacing it with a reference to "three and four tenths percent
          (3.4%)";

                                       7
<PAGE>

          (c) Section 7.4 of the Participation Agreement is amended by deleting
     it in its entirety and replacing it with the following:

          "7.4.  Facility Fee.

               Until the Maturity Date, the Lessee agrees to pay or to cause to
          be paid to the Agent for the account of (a) the Lenders, respectively,
          a facility fee (the "Lender Facility Fee") equal to the product of
          either (i) the average daily Commitment of each Lender (for Tranche A
          Loans and Tranche B Loans) during the Commitment Period or (ii) after
          the Commitment Period, the average daily aggregate amount of the
          Advances of such Lender outstanding until the Maturity Date, in each
          case multiplied by the applicable percentage set forth in the matrix
          below per annum and (b) the Holders, respectively, a facility fee (the
          "Holder Facility Fee") equal to the product of either (i) the average
          daily Holder Commitment of each Holder during the Commitment Period or
          (ii) after the Commitment Period, the average daily aggregate amount
          of the Advances of such Holder outstanding until the Maturity Date, in
          each case multiplied by the applicable percentage set forth in the
          matrix below per annum. From the Effective Date until the fifth (5th)
          day after delivery of the first financial statements and Compliance
          Certificate pursuant to Section 28.1 of the Lease following the
          consummation of the Merger (which financial statements and Compliance
          Certificate shall be prepared on a pro forma basis as if the Merger
          had occurred on the first day of such quarter), such applicable
          percentage shall be 0.275% for both the Lender Facility Fee and for
          the Holder Facility Fee. Thereafter, such Facility Fees shall be
          calculated on the basis of a year of three hundred sixty (360) days
          for the actual days elapsed and shall be payable in arrears on the
          last Business Day of each fiscal quarter of the Lessee, commencing
          with the fiscal quarter first ending after the Effective Date. The
          Facility Fees shall be reset from time to time at the time of the
          resetting of the Applicable Margin. If all or a portion of any such
          Facility Fee shall not be paid when due, such overdue amount shall
          bear interest, payable by the Lessee on demand, at a rate per annum
          equal to the ABR (or in the case of Holder Yield, the ABR plus the
          Applicable Margin for Eurodollar Holder Advances) plus two percent
          (2%) from the date of such non-payment until such amount is paid in
          full (as well as before judgment).

<TABLE>
<CAPTION>
                   Ratio of Consolidated                              Lender                Holder
                 Debt to Annualized EBITDA                         Facility Fee          Facility Fee
                ---------------------------                        ------------          ------------
<S>                                                                <C>                   <C>
Greater than 3.00 to 1.00                                              0.325%                0.325%

Greater than 2.50 to 1.00 but less than or equal to 3.00               0.300%                0.300%
 to 1.00

Greater than 2.00 to 1.00 but less than or equal to 2.50               0.300%                0.300%
 to 1.00
</TABLE>

                                       8
<PAGE>

<TABLE>
<CAPTION>
                   Ratio of Consolidated                              Lender                Holder
                 Debt to Annualized EBITDA                         Facility Fee          Facility Fee
                ---------------------------                        ------------          ------------
<S>                                                                <C>                   <C>
Greater than 1.50 to 1.00 but less than or equal to 2.00               0.275%                0.275%
 to 1.00

Greater than 1.00 to 1.00 but less than or equal to 1.50               0.275%                0.275%
 to 1.00

Less than or equal to 1.00 to 1.00                                     0.250%                0.250%"
</TABLE>

     (d) Section 8.7(b)(iii) of the Participation Agreement is deleted in its
entirety and replaced with the following"

          "(iii) Subject to Section 8.7(c), an amount equal to any payment
     identified as proceeds of the sale or other disposition (or lease upon the
     exercise of remedies) of the Properties or any portion thereof, whether
     pursuant to Article XXII of the Lease or the exercise of remedies under the
     Security Documents or otherwise, the execution of remedies set forth in the
     Lease and any payment in respect of excess wear and tear pursuant to
     Section 22.3 of the Lease (whether such payment relates to a period before
     or after the Construction Period Termination Date) shall be applied and
     allocated by the Agent first, ratably to the payment of the principal and
     interest of the Tranche B Loans then outstanding, second, ratably to the
     payment of the principal and interest of the Tranche A Loans then
     outstanding, third, to the extent such amount exceeds the maximum amount to
     be returned pursuant to the foregoing provisions of this paragraph (iii),
     ratably to the payment to the Holders of the outstanding principal balance
     of all Holder Advances plus all outstanding Holder Yield with respect to
     such outstanding Holder Advances, fourth, to any and all other amounts
     owing under the Operative Agreements to the Lenders under the Tranche B
     Loans, fifth, to any and all other amounts owing under the Operative
     Agreements to the Lenders under the Tranche A Loans, sixth, to any and all
     other amounts owing under the Operative Agreements to the Holders, and
     seventh, to the extent moneys remain after application and allocation
     pursuant to clauses first through sixth above, to the Owner Trustee for
     application and allocation to any and all other amounts owing to the
     Holders or the Owner Trustee and as the Holders shall determine; provided,
     where no Event of Default shall exist and be continuing and a prepayment is
     made for any reason with respect to less than the full amount of the
     outstanding principal amount of the Loans and the outstanding Holder
     Advances, the proceeds shall be applied and allocated ratably to the
     Lenders and to the Holders."

     (e) The first sentence of Section 10.1 is deleted and replaced with
the following sentence:

                                       9
<PAGE>

               "Each Lender may participate, assign or transfer all or a portion
          of its interest hereunder and under the other Operative Agreements in
          accordance with Sections 9.7 and 9.8 of the Credit Agreement;
          provided, at such time each participant, assignee or transferee must
          obtain an equal percentage of such Lender's interest in the Available
          Commitments for Tranche A Loans, all outstanding Tranche A Loans, the
          Available Commitments for Tranche B Loans and all outstanding Tranche
          B Loans and in the Lessee Credit Agreement; provided, further, any
          such participation, assignment or transfer of an interest under the
          Lessee Credit Agreement must also satisfy the requirements of the
          Lessee Credit Agreement.  (As an example, if a Lender holds under the
          Lessee Credit Agreement, a commitment percentage of 12.5% of each
          facility thereunder and under the Credit Agreement a commitment
          percentage of 12.0% regarding each of the Tranche A Commitments and
          the Tranche B Commitments and if such Lender desires to sell an
          assignment of 20.0% of its interests under the Lessee Credit Agreement
          and the Credit Agreement, then such Lender shall sell under the Lessee
          Credit Agreement an interest of 2.5% of all available commitments and
          of all outstandings thereunder, assuming all other requirements are
          met thereunder, and under the Credit Agreement an interest of 2.4% of
          all of the Available Commitments for Tranche A Loans and all
          outstanding Tranche A Loans and an interest of 2.4% of all of the
          Available Commitments for Tranche B Loans and all outstanding Tranche
          B Loans.)" and

          (f) A new Section 11.6 is added as follows:

     "11.6.  INDEMNITY PRIOR TO COMPLETION DATE / CONSTRUCTION PERIOD
             TERMINATION DATE.

  NOTWITHSTANDING THE PROVISIONS OF SECTIONS 11.1, 11.2, 11.3, 11.4 AND 11.5,
THE OWNER TRUSTEE SHALL BE THE ONLY BENEFICIARY OF THE PROVISIONS SET FORTH IN
SECTIONS 11.1, 11.2, 11.3, 11.4 AND 11.5 WITH RESPECT TO ANY CLAIM ARISING
THEREUNDER FOR THE PERIOD PRIOR TO THE EARLIER TO OCCUR OF THE APPLICABLE
COMPLETION DATE AND THE CONSTRUCTION PERIOD TERMINATION DATE RELATED TO THE
APPLICABLE PROPERTY.  NOTWITHSTANDING THE FOREGOING, TO THE EXTENT THAT THE
OWNER TRUSTEE BECOMES OBLIGATED TO ANY INDEMNIFIED PERSON PURSUANT TO THE NEXT
SUCCEEDING PARAGRAPH OF SECTION 11.6, THE OWNER TRUSTEE SHALL BE ENTITLED TO
FURTHER INDEMNITY FROM THE INDEMNITY PROVIDER UNDER SECTIONS 11.1, 11.2, 11.3,
11.4 AND 11.5, AS APPLICABLE, WITH RESPECT TO ALL AMOUNTS OWING OR PAID BY IT
UNDER THIS SECTION 11.6.

  TO THE EXTENT THE INDEMNITY PROVIDER IS NOT OBLIGATED TO INDEMNIFY ANY
INDEMNIFIED PERSON WITH RESPECT TO CLAIMS ARISING UNDER SECTIONS 11.1, 11.2,
11.3, 11.4 OR 11.5, PRIOR TO THE EARLIER TO OCCUR OF THE APPLICABLE COMPLETION
DATE OR CONSTRUCTION PERIOD TERMINATION DATE, THE OWNER TRUSTEE SHALL PROVIDE
SUCH INDEMNITIES IN FAVOR OF SUCH INDEMNIFIED PERSON IN ACCORDANCE WITH THE
RELEVANT PROVISIONS OF SECTIONS 11.1, 11.2, 11.3, 11.4 OR 11.5 AS THE CASE MAY
BE.  IT IS ACKNOWLEDGED AND AGREED THAT ANY AMOUNT FOR WHICH THE OWNER TRUSTEE
BECOMES OBLIGATED TO ANY INDEMNIFIED PERSON PURSUANT HERETO SHALL BECOME A CLAIM
FOR WHICH THE OWNER TRUSTEE IS ENTITLED TO INDEMNITY FROM THE INDEMNITY
PROVIDER.

                                       10
<PAGE>

    THE INDEMNITY OBLIGATIONS UNDERTAKEN BY THE OWNER TRUSTEE PURSUANT TO THIS
SECTION 11.6 ARE IN ALL RESPECTS SUBJECT TO THE LIMITATIONS ON LIABILITY
REFERENCED IN SECTION 12.9."

4.  Appendix A to the Participation Agreement.  Appendix A to the Participation
Agreement is amended as follows:

          (a) by deleting the phrase "in no event shall the Expiration Date be
     later than the fifth annual anniversary of the Initial Closing Date," from
     the definition of Expiration Date and replacing it with the following: "in
     no event shall the Expiration Date be later than the fifth annual
     anniversary of the Effective Date,";

          (b) by adding the following definitions in appropriate alphabetical
     order:

          ""Direct Ground Lease" shall have the meaning specified in the Third
          Amendment.";

          ""Effective Date" shall have the meaning specified in the Third
          Amendment.";

          ""Indirect Ground Lease" shall have the meaning specified in the Third
          Amendment.";

          ""Land Cost" shall have the meaning specified in Section 5.4 of the
          Agency Agreement.";

          ""Maximum Amount" shall have the meaning specified in Section 5.4 of
          the Agency Agreement.";

          ""Merger" shall mean the merger of Physician Reliance Network, Inc.
          with and into Diagnostic Acquisition, Inc. pursuant to that certain
          Agreement and Plan of Merger by and among Physician Reliance Network,
          Inc., American Oncology Resources, Inc. and Diagnostic Acquisition,
          Inc., dated as of December 11, 1998."

          ""Third Amendment" shall mean the Third Amendment to Certain Operative
          Agreements dated as of May 14, 1999 by and among the parties to the
          Participation Agreement as of such date, to be effective on the
          Effective Date.

          ""Total Commitments" shall mean the aggregate of the Lender
          Commitments and the Holder Commitments.";

          (c) by deleting the reference to "$2,250,000;" in the definition of
     Holder Commitment and replacing it with the following: "the aggregate
     committed amount set forth in Schedule I to the Trust Agreement, as such
     Schedule I may be amended and replaced from time to time;";

                                       11
<PAGE>

          (d) by deleting the reference to "$72,750,000;" in the definition of
     Lender Commitment and replacing it with the following: "the aggregate
     committed amount set forth in Schedule 1.1 to the Credit Agreement, as such
     Schedule 1.1 may be amended and replaced from time to time;";

          (e) by deleting the text after "(ii)" in the definition of
     Construction Period Termination Date and replacing it with the following:
     "the second anniversary of the Effective Date or (b) such later date as
     shall be agreed by the Majority Secured Parties.";

          (f) The definition for "Applicable Margin" is deleted in its entirety
     and replaced with the following:

               ""Applicable Margin" shall mean, at any time with respect to any
          Eurodollar Loan or any Eurodollar Holder Advances, the applicable
          percentage points as determined under the following matrix with
          reference to the ratio of Consolidated Debt to Annualized EBITDA
          calculated as provided below:

<TABLE>
<CAPTION>
             Ratio of Consolidated                 Applicable Margin for    Applicable Margin for    Applicable Margins for
                    Debt to                              Eurodollar               Eurodollar               Eurodollar
               Annualized EBITDA                      Tranche A Loans          Tranche B Loans           Holder Advances
             ----------------------                ----------------------   ----------------------   -----------------------
<S>                                                <C>                      <C>                      <C>
Greater than 3.00 to 1.00                                   1.550%                   1.925%                    2.300%

Greater than 2.50 to 1.00 but less than or                  1.450%                   1.825%                    2.200%
 equal to 3.00 to 1.00

Greater than 2.00 to 1.00 but less than or                  1.325%                   1.700%                    2.075%
 equal to 2.50 to 1.00

Greater than 1.50 to 1.00 but less than or                  1.225%                   1.600%                    1.975%
 equal to 2.00 to 1.00

Greater than 1.00 to 1.00 but less than or                  1.100%                   1.475%                    1.850%
 equal to 1.50 to 1.00

Less than or equal to 1.00 to 1.00                          1.000%                   1.375%                    1.750%
</TABLE>

               From the Effective Date until the fifth (5th) day after receipt
          by the Agent of the first financial statements and Compliance
          Certificate pursuant to Section 28.1 of the Lease following the
          consummation of the Merger (which financial statements and Compliance
          Certificate shall be prepared on a pro forma basis as if the Merger
          had occurred on the first day of such quarter), the Applicable Margin
          shall be 1.225% (for the Eurodollar Tranche A Loans), 1.600% (for the
          Eurodollar Tranche B Loans) and 1.975% (for the Eurodollar Holder
          Advances). Thereafter, the Applicable Margin shall be reset from time
          to time in accordance with the above matrix on the fifth (5th) day
          after receipt by the Agent in accordance with Section 28.1 of the
          Lease of financial statements together with a Compliance Certificate
          (reflecting the computation of the ratio of Consolidated Debt to

                                       12
<PAGE>

          Annualized EBITDA as of the last day of the preceding fiscal quarter
          or fiscal year, as appropriate)."; and

          (g) The definition for "Lessee Credit Agreement" is amended by
     deleting it in its entirety and replacing it with the following:

               ""Lessee Credit Agreement" shall mean that certain Fourth Amended
          and Restated Loan Agreement dated as of the date of the Third
          Amendment among AOR, the various lenders parties thereto from time to
          time and First Union National Bank, as the agent thereunder, as such
          may hereafter be amended, modified, supplemented, restated and/or
          refinanced or otherwise replaced from time to time."

  5.  Trust Agreement.  The Trust Agreement is amended as follows:

          (a) Exhibit A to the Trust Agreement is amended by deleting it in its
     entirety and replacing it with Exhibit A attached hereto; and

          (b) Schedule I to the Trust Agreement is amended by deleting it in its
     entirety and replacing it with Exhibit B attached hereto.

  6.  Credit Agreement.  The Credit Agreement is amended as follows:

          (a) Section 2.5(a) of the Credit Agreement is amended by deleting the
     reference to "ninety-seven percent (97%)" and replacing it with a reference
     to "ninety-six and six tenths percent (96.6%)";

          (b) Exhibit A-1 and Exhibit A-2 to the Credit Agreement are amended by
     deleting them in their entirety and replacing them respectively with
     Exhibit C-1 and Exhibit C-2 attached hereto; and

          (c) Schedule 1.1 to the Credit Agreement is amended by deleting it in
     its entirety and replacing it with Exhibit D attached hereto.

  7.  Lease Agreement.  The Lease Agreement is amended as follows:

          (a) The first paragraph of Section 2.2 is edited by removing the
     phrase "Initial Closing Date," and replacing it with the following:
     "Effective Date," and

          (b) The first sentence of the second paragraph of Section 2.2 of the
     Lease is amended by deleting the reference to "Initial Closing Date" and
     replacing it with a reference to "Effective Date".

  8.  Security Agreement.  The Preliminary Statement of the Security Agreement
is amended by deleting the references to "$72,750,000" and "$2,250,000" and
replacing them with

                                       13
<PAGE>

references to "the aggregate Lender Commitments from time to time" and "the
aggregate Holder Commitments from time to time" respectively.

  9.  Effective Date.  The effective date for this Amendment (the "Effective
Date") shall be the date as of which the following conditions precedent have
been satisfied or waived by the Agent (provided, that any such waiver shall
require subsequent satisfaction (unless expressly stated to be a permanent
waiver) of any such condition so waived; provided, further, if the Effective
Date does not occur prior to August 1, 1999 then there shall be no Effective
Date and none of the Assignments nor this Amendment shall have any further force
or effect; provided, further, from and after the Effective Date, the obligations
of the Credit Parties shall not be subject to any conditions contained in this
Section 9 which are required to be performed by any Credit Party):

          (a) The Fourth Amended and Restated Loan Agreement dated as of or
     about the date hereof among First Union National Bank, as administrative
     agent thereunder, First Chicago Capital Markets, Inc., as syndication agent
     thereunder, the various lenders thereunder and American Oncology Resources,
     Inc., as borrower thereunder, shall be in full force and effect;

          (b) All entities subject to the requirements of Section 5.8 of the
     Participation Agreement prior to the Effective Date shall have satisfied in
     full all such requirements;

          (c) Each of the Lessee, the Guarantor and each Tranche A Guarantor
     shall have delivered or caused to be delivered to the Agent (i) copies of
     good standing certificates or certificates of existence from their
     respective states of incorporation and their respective principal places of
     business, (ii) an Officer's Certificate of all of the Credit Parties
     certifying as to the correctness of the representations and warranties of
     each Credit Party set forth in the Operative Agreements (except for any
     such representations and warranties which relate solely to an earlier time)
     and certifying that no Default or Event of Default shall have occurred and
     be continuing, (iii) secretary's certificates regarding charter documents,
     by-laws, resolutions and incumbency and (iv) an opinion of counsel, in each
     case with respect to the foregoing subsections (i), (ii), (iii) and (iv) in
     form and substance reasonably satisfactory to the Agent;

          (d) The Lessee shall have delivered or caused to be delivered to the
     Agent such of the following as shall be reasonably requested by the Agent:
     (i) amendments to Uniform Commercial Code financing statements and fixture
     filings, (ii) amendments to the memoranda of Lease and Lease Supplements,
     (iii) amendments to the Mortgage Instruments and (iv) (A) endorsements to
     existing title policies (or binding commitments for such endorsements)
     and/or newly issued title policies (or binding commitments for such title
     policies), in each case with respect to the Properties or (B) to the extent
     Lessee shall fail to secure such endorsements or policies, opinions of
     counsel from the states in which the Properties are located, in each case
     with respect to the foregoing subsections (i), (ii), (iii) and (iv) in form
     and substance reasonably satisfactory to the Agent;

                                       14
<PAGE>

          (e) Each of the Loan Assignment to First Union, the Holder Advance
     Assignment to First Union, the Loan Assignment to the New Lenders and the
     Holder Advance Assignment to the New Holders (in each case in form and
     substance reasonably satisfactory to the Agent) shall have been delivered
     to the Agent and shall be in full force and effect;

          (f) Replacement Certificates and Notes in favor of the Holders and
     Lenders, respectively, shall have been delivered to the Agent and shall be
     in full force and effect;

          (g) The Lessee shall have delivered no less than three (3) Business
     Days notice to the Agent identifying the specific date as of which the
     Effective Date shall occur, and the Agent shall be responsible for promptly
     forwarding such notice to each Lender, Holder, Prior Lender and Prior
     Holder; and

          (h) Any other documents or items in connection with matters relating
     to this Amendment, reasonably required by the Agent, shall have been
     delivered to the Agent.

  10.  Ground Leases.  The Operative Agreements provide that a Property may be
subject to a Ground Lease provided the Ground Lease is in a form and substance
satisfactory to the Agent.  The parties hereto acknowledge that a proposed
Ground Lease may either be a direct Ground Lease from a ground lessor to the
Lessor (a "Direct Ground Lease") or a Ground Lease from a ground lessor to the
Lessee (or any Subsidiary of any Credit Party) that is subsequently sub-ground
leased to the Lessor (an "Indirect Ground Lease").  All references in the
Operative Agreements to a Ground Lease shall be deemed to be a reference to
either a Direct Ground Lease or an Indirect Ground Lease, as applicable.

  11.  Counterparts.  This Amendment may be executed in any number of
counterparts, each of which when executed and delivered shall be deemed to be an
original and it shall not be necessary in making proof of this Amendment to
produce or account for more than one such counterpart provided, that to the
extent this Amendment constitutes chattel paper (as such term is defined in the
Uniform Commercial Code), no Lien on this Amendment may be created or perfected
through the transfer or possession of any counterpart hereof other than the
counterpart bearing the receipt therefor executed by the Agent on the signature
page hereof, which counterpart shall constitute the only "original" hereof for
purposes of the Uniform Commercial Code.

  12.  Continued Effectiveness of Operative Agreements.  Except as modified
hereby, all of the terms and conditions of the Operative Agreements shall remain
in full force and effect.

  13.  Governing Law.  This Amendment shall be governed by, and construed in
accordance with, the laws of the State of North Carolina except as to Section 5
of this Amendment and the Trust Agreement which shall be governed by, and
construed in accordance with, the laws of the State of Utah.

                                       15
<PAGE>

  14.  No Novation.  After the effective date of the Assignments and after the
effective date of this Amendment, the parties to this Amendment intend for the
Loans, the Holder Advances and the other obligations evidenced by the Operative
Agreements to constitute the same Loans, Holder Advances and other such
obligations that existed prior to the effective date of the Assignments and
prior to the effective date of this Amendment and not to constitute a novation.

  15.  Ratification.  Although not required due to the authority granted to the
Agent pursuant to Section 8.6 of the Participation Agreement and Section 7 of
the Credit Agreement, each Holder and each Lender which is a party to this
Amendment hereby ratifies the actions of the Agent regarding the Agent's
execution and delivery of the amendments to the Mortgage Instruments and all
matters related thereto and/or to this Amendment.



        [The remainder of this page has been intentionally left blank.]

                                       16
<PAGE>

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

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

                    AMERICAN ONCOLOGY RESOURCES, INC.,
                    as the Guarantor and as a Tranche A Guarantor

                    AOR REAL ESTATE, INC., as a Tranche A Guarantor

                    AOR, 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

                          [SIGNATURE PAGES CONTINUED]

                                       17
<PAGE>

                    AOR MANAGEMENT COMPANY OF CENTRAL FLORIDA, INC.,
                    as a Tranche A Guarantor

                    AOR MANAGEMENT COMPANY OF OHIO, INC.,
                    as a Tranche A Guarantor

                    AOR MANAGEMENT COMPANY OF KANSAS, INC.,
                    as a Tranche A Guarantor

                    AORT HOLDING COMPANY, INC., as a Tranche A Guarantor

                    AORIP, INC., as a Tranche A Guarantor

                    GREENVILLE RADIATION CARE, INC.,
                    as a Tranche A Guarantor

                    By:
                       --------------------------------------------
                    Name:    L. Fred Pounds
                    Title:   Treasurer 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:    L. Fred Pounds
                    Title:   Treasurer


                    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:    L. Fred Pounds
                    Title:   Treasurer


                    By:  AOR HOLDING COMPANY OF INDIANA, INC., a Delaware
                         corporation, as general partner

                    By:
                       --------------------------------------------
                    Name:    L. Fred Pounds
                    Title:   Treasurer

                          [Signature pages continued]

                                       18
<PAGE>

                    FIRST SECURITY BANK, NATIONAL ASSOCIATION,
                    not individually, but solely as Owner Trustee under the AOR
                    Trust 1997-1


                    By:
                       --------------------------------------------
                    Name:
                          -----------------------------------------
                    Title:
                          -----------------------------------------

                          [Signature pages continued]

                                       19
<PAGE>

                    FIRST UNION NATIONAL BANK, as a Lender
                    and as a Holder


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                         ------------------------------------------


                          [SIGNATURE PAGES CONTINUED]


                                       20
<PAGE>

                    THE FIRST NATIONAL BANK OF CHICAGO,
                    as a Lender and as a Holder


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                         ------------------------------------------


                          [SIGNATURE PAGES CONTINUED]


                                       21
<PAGE>

                    COOPERATIEVE CENTRALE
                    RAIFFEISEN-BOERENLEENBANK B.A.
                    "Rabobank Nederland", NEW YORK BRANCH,
                    as a Lender and as a Holder


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          -----------------------------------------

                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          -----------------------------------------

                          [SIGNATURE PAGES CONTINUED]


                                       22
<PAGE>

                    BANKERS TRUST COMPANY,
                    as a Lender


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          -----------------------------------------


                          [SIGNATURE PAGES CONTINUED]


                                       23
<PAGE>

                    WELLS FARGO BANK (TEXAS), NATIONAL ASSOCIATION,
                    as a Lender


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          -----------------------------------------


                          [SIGNATURE PAGES CONTINUED]


                                       24
<PAGE>

                    CHASE BANK OF TEXAS NATIONAL ASSOCIATION,
                    as a Lender and as a Holder


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          -----------------------------------------


                          [SIGNATURE PAGES CONTINUED]


                                       25
<PAGE>

                    NATIONSBANK, N.A.,
                    as a Lender


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          -----------------------------------------


                          [SIGNATURE PAGES CONTINUED]


                                       26
<PAGE>

                    CREDIT LYONNAIS NEW YORK BRANCH,
                    as a Lender


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          -----------------------------------------


                          [SIGNATURE PAGES CONTINUED]


                                       27
<PAGE>

                    U.S. BANK NATIONAL ASSOCIATION,
                    as a Lender


                    By:
                       --------------------------------------------
                    Name:
                         ------------------------------------------
                    Title:
                          ----------------------------------------


                           [SIGNATURE PAGES CONTINUED

                                       28
<PAGE>

     Receipt of the original counterpart of the foregoing Amendment is hereby
acknowledged on this ____ day of ______________, 1999./1/


                                   FIRST UNION NATIONAL BANK, as Agent


                                   By:
                                      ----------------------------------
                                      Name:
                                           -----------------------------
                                      Title:
                                            ----------------------------
------------
/1/ This acknowledgment is executed in the original counterpart only.



                                       29