A potential move had looked in doubt earlier this week when discussions with Marseille broke down, but Barton travelled to France on Wednesday for further talks with the Ligue 1 club.
It appears those have gone more smoothly, with the midfielder now in a medical with the Stade Velodrome side.
Marseille coach Elie Baup has confirmed the club are in talks with the midfielder, but stressed that a move has yet to be agreed.
Baup told OM's website: "Joey Barton is in the building. I don't know if it will be finalised today or how it will turn out."
Baup had praise for Barton's qualities as a player but warned that if he is to become part of the set-up at the Stade Velodrome then the controversial Liverpudlian must fit into the team ethic.
"He is someone who all the world knows," he said.
"He is an English box-to-box midfielder, he works hard on the pitch and can win the ball back well. He also uses the ball well.
"This is the type of player that allows a team to play at a high tempo and to quickly recover when they lose the ball.
"He has to be part of the project, respect the team rules and work in the greater interest of the team. He must comply with these standards.
"His career indicates that he knows that already. And a player of character is often necessary."
The 29-year-old's relationship with QPR has deteriorated rapidly over the summer after his behaviour in the match against Manchester City on the final day of last season saw him charged with violent conduct and banned for 12 games.
The Hoops have frozen out the once-capped England international at Loftus Road, stripping him of the captain's armband and not giving him a squad number for the new campaign.
Barton has made it clear he wants to leave West London for Marseille and QPR have said they will allow him to leave on a free transfer.
