Ligue 1 title contenders Marseille were beaten 3-1 at 10-man Sochaux on Sunday, while Nice thrashed Valenciennes 5-0 and Bastia won at Reims.
OM returned from the winter break level at the top with Paris Saint-Germain and Lyon but are adrift of both after a forgettable night in the snow in Montbeliard.
First-half goals from Sloan Privat and Cedric Bakambu stunned the visitors before a Jeremy Morel own goal shortly after half-time handed Sochaux full control.
Their grip loosened when Jordan Ayew quickly replied and then Joseph Lopy saw red on the hour.
But Marseille failed to make any further inroads with Lucas Mendes' header against the post with the final touch the closest they came.
The blistering form Nice showed before the Christmas break continued as Claude Puel's thrashed Valenciennes 5-0.
Les Aiglons had to wait until the final minute of the first half to take the lead, Dario Cvitanich doing the trick, but the goals flowed after the break, with Cvitanich getting his second while Renato Civelli, Valentin Eysseric and Didier Digard also chipped in.
Reims remained in relegation trouble as they went down 2-1 at home to Bastia.
The visitors grabbed a two-goal lead thanks to Ilan's opener and Florian Thauvin's effort just after half-time.
Gaetan Courtet gave Reims hope shortly after the hour mark, but their failure to find a leveller leaves them outside the drop zone on goal difference only.