The 29-year-old was slapped with a 12-match ban following his sending off on the final day of the season at Manchester City and subsequent attacks on Sergio Aguero and Vincent Kompany.
|40/1||QPR are 40/1 with Bet365 to beat Man City 2-1 on Saturday
Barton was also stripped of the Hoops' captaincy and was not assigned a squad number for the current campaign.
Barton expressed his delight at the move on Friday 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
The move follows comments from Barton on Wednesday saying that he could not remain at QPR after his relationship with the club broke down to the point where he wasn't given a squad number for the 2012-13 season.
"It's not a great position for me to be in as a professional when you feel unwanted, unloved and you haven't got a shared number never mind a locker at the training ground," he told Sky Sports News.