PROSPECTUS SUPPLEMENT
(To Prospectus Dated March 11, 1997)

                               OMNICOM GROUP INC.
                        $218,500,000 Principal Amount of
               4 1/4% Convertible Subordinated Debentures due 2007
                     (Interest Payable January 3 and July 3)

                        3,468,254 Shares of Common Stock

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     This document  supplements the Prospectus  dated March 11, 1997 relating to
(i) $218,500,000  aggregate principal amount of 4 1/4% Convertible  Subordinated
Debentures  due 2007  (the  "Debentures")  of  Omnicom  Group  Inc.,  a New York
corporation  ("Omnicom" or the "Company),  and (ii)  3,468,254  shares of Common
Stock,  par value $0.50 per share (the "Common  Stock") of the Company which are
initially   issuable  upon   conversion  of  the  Debentures   plus   additional
indeterminate  number  of shares of Common  Stock as may  become  issuable  upon
conversion of the Debentures as a result of adjustments to the conversion  price
(the  "Shares").  The  Debentures  were  initially  acquired from the Company by
Morgan Stanley & Co.  Incorporated  in January 1997 in connection with a private
offering.  This  Prospectus  Supplement is  incorporated  by reference  into the
Prospectus, and all terms used herein shall have the meaning assigned to them in
the  Prospectus.  On May 19,  1997  the  closing  price of the  Common  Stock as
reported on the New York Stock Exchange was $55 3/8 per share.  The Common Stock
is traded under the symbol "OMC".

     The Company hereby  clarifies the description of the Debentures by deleting
the second  sentence of the third  paragraph  under the heading  "General" which
appears on page 5 of the Prospectus, and replacing with it a sentence that reads
as follows:

     "Debentures  which are repaid at the  option of a holder  have a yield from
     the original issue date to the Holder  Repayment Date (as defined below) of
     6% (including semiannual cash interest payments and repayment premium)."

     In addition,  in  accordance  with the Section of the  Prospectus  entitled
"Selling  Securityholders" (which appears on pages 19 and 20 of the Prospectus),
the following  information is provided with respect to the beneficial  owners of
the Debentures:

                                  Principal Amount          Principal Amount of
      Name of Selling               of Debentures             Debentures to be
     Securityholder(s):           Beneficially Owned          Offered for Sale
     ------------------           ------------------          ----------------
McMahan Securities Company, L.P.      $3,500,000                 $3,500,000

     The Debentures being offered by the Selling Securityholder hereby represent
all of the Debentures beneficially owned by the Selling Securityholder as of May
15, 1997. Except for the purchase of the Debentures,  the Selling Securityholder
has not had a material  relationship  with the Company or any of its  affiliates
within the past three years.

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     THESE  SECURITIES  HAVE NOT BEEN APPROVED OR  DISAPPROVED BY THE SECURITIES
AND EXCHANGE  COMMISSION OR ANY STATE SECURITY COMMISSION NOR HAS THE SECURITIES
AND  EXCHANGE  COMMISSION  OR ANY STATE  SECURITIES  COMMISSION  PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

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            The date of this Prospectus Supplement is May 20, 1997.