The co-chief executives, Bill Gray and Carla Hendra, previously ran the North America set-up. Gray will now become the vice-chairman of the region, while Hendra will continue in the chief executive role, reporting to Seifert.
Seifert was previously the chairman of Ogilvy's global brand community, overseeing 25 of its major global clients.
Miles Young, the Ogilvy worldwide chief executive, said: "North America is our most important region.
"The changes provide the clarity of leadership which the new Ogilvy I envisage requires."