29-year old Barton has joined the French club on a season-long loan whilst 26-year old Cameroon midfielder Mbia has signed a two-year deal with the R's.
"I am now a player for L'OM. What a birthday present! Cannot wait to get going. #allezL'OM," Barton tweeted.
"I would also like to thank @tonyfernandes and @Amit_Bhatia99 for making this possible. Good luck to everyone at QPR for this season."