Exhibit 99.1
                                CERTIFICATION OF
                                QUARTERLY REPORT
                                  ON FORM 10-Q

Pursuant  to  18  U.S.C.   ss.1350,   as  adopted  pursuant  to  ss.906  of  the
Sarbanes-Oxley  Act of 2002,  in  connection  with the filing of Omnicom  Group,
Inc.'s  Quarterly  Report on Form 10-Q for the quarter  ended March 31, 2003, as
filed with the  Securities  and  Exchange  Commission  on the date  hereof  (the
"Report"), each of the undersigned officers of Omnicom certifies,  that, to such
officer's knowledge:

      o     the Report fully complies with the  requirements of Section 13(a) or
            15(d) of the Securities Exchange Act of 1934; and

      o     the  information  contained in the Report  fairly  presents,  in all
            material aspects,  the financial condition and results of operations
            of  Omnicom  as of the dates and for the  periods  expressed  in the
            Report.


Executed as of May 15, 2003.


                                  /s/ John D. Wren
                                  ---------------------------------------------
                                  Name:  John D. Wren
                                  Title: Chief Executive Officer and President



                                  /s/ Randall J. Weisenburger
                                  ---------------------------------------------
                                  Name:  Randall J. Weisenburger
                                  Title: Executive Vice President and
                                         Chief Financial Officer