[Letterhead of Mayor, Day, Caldwell & Keeton, L.L.P.]

                                                                       EXHIBIT 5
 
                                 May 10, 1999


American Oncology Resources, Inc.
16825 Northchase Drive, Suite 1300
Houston, Texas 77060

Ladies and Gentlemen:

  We have acted as counsel for American Oncology Resources, Inc., a Delaware
corporation (the "Company"), in connection with the registration and proposed
issuance of up to an aggregate of 48,800,000 shares (the "Shares") of Common
Stock, par value $.01 per share, of the Company ("Common Stock"), to be issued
in connection with a proposed merger (the "Merger") of Diagnostic Acquistion,
Inc., a Delaware corporation ("Sub"), that is the Company's wholly owned
subsidiary, with and into Physician Reliance Network, Inc. ("PRN"), a Delaware
corporation, all as described in the Company's Registration Statement on Form 
S-4 filed on even date herewith with the Securities and Exchange Commission
under the Securities Act of 1933, as amended ("Registration Statement"). In such
capacity, we have familiarized ourselves with the Articles of Incorporation and
Bylaws of the Company and have examined all statutes and other records,
instruments and documents pertaining to the Company that we have deemed
necessary to examine for the purposes of this opinion.

  Based upon our examination as aforesaid, we are of the opinion that:

     1.   The Company is a corporation duly incorporated, validly existing and
          in good standing under the laws of the State of Delaware; and

     2.   Upon the issuance of the up to 50,000,000 Shares being issued by the
          Company in the Merger approved by the Company's Board of Directors and
          in accordance with the terms of the Merger Agreement (as defined in
          the Registration Statement) among the Company, Sub and PRN, such
          Company Shares will be duly authorized, validly issued, fully paid and
          nonassessable shares of Common Stock.

  We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an exhibit to the Registration Statement and to the use
of our name in the Registration Statement at each place in which it appears.

                                       Very truly yours,

                                       /s/ MAYOR, DAY, CALDWELL & KEETON, L.L.P.