Clinton Njie and Dmitri Payet found the net to put OM on 44 points from 21 games, nine points behind runaway leaders Paris St Germain, who take on Dijon on Wednesday.
Monaco are third on 43 points after Mario Balotelli's double earned Nice a 2-2 draw at the Louis II stadium in the Azurean derby.
Marseille struggled to break the deadlock in a dull first half but found a way through after the break.
Valere Germain missed a penalty in the 73rd minute but the home side were rewarded for their dominance 10 minutes from fulltime when Njie volleyed home from close range.
Payet put the result beyond doubt in the 87th, poking the ball home after dribbling past keeper Alexandre Oukidja.
Champions Monaco took the lead against the run of play after 33 minutes as Adama Diakhaby scored from point-blank range from Thomas Lemar's cross.
Yet Nice equalised through Balotelli two minutes into the second half when the Italian latched onto Allan Saint Maximin's cross from the right.
Balotelli put Nice ahead in the 68th, easing his way past Andrea Raggi before beating Danijel Subasic from close range.
It was the Italian's third double in four games against Monaco.
Radamel Falcao salvaged a point for the hosts with a goal two minutes into stoppage time.
(Reporting by Julien Pretot; Editing by Toby Davis)
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