Montpellier's troubled title defence continued as Marseille maintained their 100% start to the Ligue 1 season with a narrow win at Stade de la Mosson.
Andre-Pierre Gignac struck the winning goal in the 77th minute, capitalising on an error in the champions' backline, to make it three wins from three under new OM boss Elie Baup.
Defeat for Montpellier leaves them just one place above the relegation zone and still searching for their first win after three matches.
Marseille just edged a cagey first half played in punishing temperatures, with Gignac twice coming close to breaking the deadlock but home goalkeeper Geoffrey Jourdren was equal to both efforts.
Montpellier's best chance of the opening 45 minutes fell to Daniel Congre after OM's wall had charged down a free-kick, but the defender dragged his shot wide.
Jourdren was again in action a minute after the restart as he punched clear Mathieu Valbuena's free-kick.
The goalkeepers continued to play a central role as the second half wore on, with Jourdren keeping out a Gignac free-kick in the 55th minute, while opposite number Steve Mandanda first denied John Utaka and then Romain Pitau.
But with the clock ticking down the goal finally arrived, with the visitors indebted to two errors by the home defence for the breakthrough.
First Congre's poor clearance was pounced on by Morgan Amalfitano, and then when Gignac was played in by his team-mate, the striker's effort from the edge of the area passed under the body of the hitherto impressive Jourdren to seal all the points.