And the move gives Publicis, which has faced questions about who will succeed 71-year-old Levy, access to Omnicom's well-regarded senior leadership, said James Dix, an analyst at Wedbush Securities.
A major concern, though, is whether the two companies can strike a harmonious balance of power -- something that can be difficult in mergers of similar-sized companies based in different countries.
"You have these fiefdoms that keep people from playing together," Dix said in an interview Saturday. "One company is based in Paris, one is in New York. Where is the power center?"
Dix expects that top executives are comfortable with the structure of the deal, but the adjustment may be more difficult for the next level of executives who run the firms' units.
"Now they have to fit together into a broader organization," Dix said. "If you lose clients or have defections of senior executives then you have something that looked good on paper but didn't quite play out."
Business News reporter Jon Fahey contributed from New York.