OMD's European chairman, Viviane Prat, is leaving the company at the beginning of next year, though she will be retained as a consultant to Omnicom shareholders.
Prat, who is also the president of OMD in France, is expected to step aside early in the New Year and a replacement is now being sought to head the French office.
The wider European management structure will come under scrutiny once a new global chief of OMD is in place. The Omnicom media network has been searching for a new head since the summer, when Daryl Simm was promoted to a broader group media role within the Omnicom empire.
In the meantime, OMD Europe's chief operating officer, Johan Denekamp, will continue to steer the European network.
Prat is understood to have decided to resign after failing to snare the pounds 360 million pan-European Renault media business and losing the McDonald's media account in France this autumn.
She took over as the head of Europe for Omnicom in 1996. However, the OMD network has found it hard to present a coherent offering and it wasn't until this year that the European offices began to embrace the OMD brand. Nineteen offices out of a total of 26 are now branded OMD.