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 tem...