The 29-year-old has left London after a difficult conclusion to the 2011-12 campaign which saw him receive a 12-match ban following a red card in QPR's 3-2 defeat to champions Manchester City on the final day of the season.
Barton expressed his delight at the move on Saturday evening by tweeting: "I am now a player for L'OM. What a birthday present! Cannot wait to get going. #allezL'OM."
The tough English midfielder also had kind words for QPR chairman Tony Fernandes and co-owner Amit Bhatia: "I would also like to thank