Each COO will report to Viacom chief executive, Tom Goddard, once the changes come into effect in January.
In the wake of Oldham's move, fellow joint managing director, Tim Bleakley, has had his role expanded and becomes UK managing director.
Clive Punter, currently joint COO of Viacom Outdoor International, will become COO of Asia; and Guillaume Prot, who currently shares the joint COO position with Punter, becomes COO of continental Europe.
Mark Pettler, Viacom Outdoor UK's delivery director, has taken the expanded role of international operations director.
Finally, Viacom has appointed Helen Goldberg as international director of legal and business affairs. She joins from Miva Media, where she was European legal director.