Andre-Pierre Gignac scored for the third match in a row as Marseille maintained their 100 per cent winning start to the Ligue 1 season after edging out Valenciennes.
Gignac is pushing for a recall to the French national set-up, having not played an international match since 2010, after a fine start to the season which continued on Saturday.
The match looked to be petering out to a goalless draw before Gignac scored from a rebound seven minutes from time to help Marseille claim three wins from as many matches at Stade du Hainaut.
Elie Baup's side were sharing first place with Lyon, Monaco and St Etienne after taking three points at Guingamp in their opening game before doubling their tally at home to Evian last weekend.
Their opponents were no pushovers and opened their campaign with a convincing 3-0 home win over Toulouse.
Despite being beaten by Bastia last week, they made a decent start against high-flying OM, whose goalkeeper Steve Mandanda reacted quickly to swoop at the feet of Jean-Christophe Bahebeck in a good position.
At the other end, Andre Ayew found Gignac but the in-form forward had yet to find his range and put his strike narrowly wide.
Ayew then nearly broke the deadlock himself but Valenciennes goalkeeper Nicolas Penneteau magnificently tipped away his header from Jeremy Morel's cross, while Mathieu Valbuena's sensational volley from 30 yards skimmed the top of the crossbar.
The hosts managed to keep the score goalless up until the interval and nearly took the lead against the run of play at the beginning of the second half, with Mandanda keeping out Bahebeck's close-range effort.
Marseille reasserted their dominance and Dimitri Mayet's 25-yard curling effort was pushed away by Penneteau, who was finally beaten in the 83rd minute.
Ayew's header from Valbuena's corner was straight at Penneteau but the goalkeeper could only block the ball into the path of the well-positioned Gignac, who tapped in the rebound with ease.
The visitors had the chance to double their lead in injury time although Saber Khalifa's fierce effort from distance was pushed away by Penneteau at full stretch.
It hardly mattered, however, as Marseille comfortably held on to walk away as deserved winners.