Marseille moved ahead of opponents Rennes and into fifth place in the Ligue 1 table after coming from behind to seal an important win at the Stade de la Route de Lorient.
Tongo Doumbia put the hosts ahead after 14 minutes, shooting low into the corner of the net after Yacine Brahimi's pass, but OM drew level just before the interval when Onyekachi Apam diverted the ball into his own net.
Benoit Cheyrou ensured the visitors left with all three points with his 77th-minute effort.
Neither side created much of note before Rennes' opener, with Lucho Gonzalez giving up possession in his own half to give Doumbia an opportunity to strike from 25 yards.
He gladly did so and a slight deflection was enough to see the ball beyond Steve Mandanda in the Marseille goal.
Mathieu Valbuena and Cheyrou combined in the 23rd minute to give Lucho the chance to make amends but he hesitated and fluffed his shot.
Morgan Amalfitano did better with his chance five minutes later, his header forcing goalkeeper Abdoulaye Diallo to palm the looping ball away for a corner.
Mandanda produced an even better save moments later from Jires Kembo Ekoko's sweetly-timed effort, while Brahimi went close from the resulting corner.
Brahimi then hit the side netting but it was Marseille who scored before the break, Loic Remy's cross met by Apam and resulting in an own-goal via the post.
Ekoko almost restored the lead four minutes into the second half but although his shot evaded the goalkeeper, Nicolas N'Koulou was on hand with a superb block.
Both sides visibly tired as the match wore on but it was the away side who found the necessary resolve to claim the points.
Valbuena provided the assist and Cheyrou did the rest, converting from the edge of the area despite Diallo getting a hand to the shot.