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

                              ---------------------

     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 July 16,  1997 the  closing  price of the  Common  Stock as
reported on the New York Stock  Exchange was $64.00 per share.  The Common Stock
is traded under the symbol "OMC".

     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 Debentures     of Debentures to be
 Name of Selling Securityholder(s):    Beneficially Owned    Offered for Sale
 ---------------------------------    --------------------  ------------------
Froley Revy Investment Co. Inc./           $ 500,000            $ 500,000
Arkansas PERS
Froley Revy Investment Co. Inc./             825,000              825,000
PRIM Board                                                   
Froley Revy Investment Co. Inc./             500,000              500,000
Southern Farm Bureau Life Insurance                          
Froley Revy Investment Co., Inc./            575,000              575,000
Regence Washington Health                                    
Froley Revy Investment Co. Inc./             200,000              200,000
Starvest Investment                                          
Froley Revy Investment Co. Inc./           4,400,000            4,400,000
State of Oregon/SAIF Corp.                                   
Froley Revy Investment Co. Inc./             215,000              215,000
Island Insurance Convertibles                                
Froley Revy Investment Co. Inc./             700,000              700,000
Finance Factors Ltd.                                         
Lazard Freres & Co. LLC                    2,630,000            2,630,000
Lutheran Brotherhood Income Fund           1,000,000            1,000,000
LB Series Fund, Inc., Income Portfolio       500,000              500,000
Lutheran Brotherhood                       3,500,000            3,500,000
Phoenix Convertible Fund                     510,000              510,000
The TCW Group, Inc.                       15,630,000           15,630,000
                                                             
                                                            
     The  Debentures  being  offered  by  the  Selling   Securityholders  hereby
represent   all  of  the   Debentures   beneficially   owned   by  the   Selling
Securityholders  as of July 17, 1997. Except for the purchase of the Debentures,
none of the Selling  Securityholders  has had a material  relationship  with the
Company or any of its affiliates within the past three years.

                             -----------------------

     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.

                             -----------------------

            The date of this Prospectus Supplement is July 18, 1997.