Robert Callander, aged 71 and a former chairman of the board's audit committee, left the company on May 22.
It is reported that he was unhappy with the group management's limited disclosure to the audit committee over internet assets.
Omnicom, headed by president and CEO John Wren, had set up a new company called Seneca, along with a private equity firm, to house its internet assets, which include Agency.com, Organic and Razorfish. Omnicom's stake in Seneca, which is a private partnership, is worth $280m (拢192m).
Shares in Omnicom fell into decline last week after rumours began to circulate that some kind of negative press story was being researched about the group, owner of the BBDO Worldwide advertising network and Fleishman-Hillard Public Relations. Speculation had it that the story may concern Omnicom's accounting practices.
On Friday, Omnicom reaffirmed its earnings expectations and said "it knew of no corporate development that accounts for the recent market activity in its stock".
However, this did not deter investors from selling on Friday, with more than 9m shares changing hands, the heaviest trading in Omnicom since 1997.
Callander's resignation was revealed in an SEC filing last Wednesday. Omnicom has recently reorganised its board, getting rid of insiders and replacing them with external advisers.
Shares closed at $72.69, down by 3.8% or $2.83, on Friday.
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