Leaders Lille could only manage a 1-1 draw against seventh-placed Lorient to hand the initiative to champions Marseille in the Ligue 1 title race..
Mathieu Debuchy's strike two minutes before half-time had looked set to give LOSC a four-point cushion at the summit, with OM due to play their game in hand at home to Nice on Wednesday.
However, Kevin Gameiro equalised midway through the second half at the Stade du Moustoir to leave Rudi Garcia's side with just a two-point advantage having picked up only two points from the last nine on offer.
Guillaume Hoarau's deflected late equaliser gave fourth-placed Paris St Germain a 2-2 draw at Brest and a potentially important point in the race for Champions League qualification.
Mathieu Bodmer had given the visitors a 20th-minute lead at the Stade Francis Le Ble, but the mid-table hosts turned the game on its head before the break through goals from Larsen Toure - following up from Bruno Grouri's saved penalty - and Grouri.
However, Hoarau's header - which went in via Ahmed Kantari - levelled matters three minutes from time to keep PSG in touch with the top three and move them two points clear of fifth-placed Rennes, who lost 1-0 at Monaco.
Rennes' Champions League hopes suffered another blow as they lost 1-0 at relegation-threatened Monaco to extend their winless Ligue 1 run to six games.
Georgie Welcome's strike 18 minutes from time was enough to secure victory for the hosts, who started the day outside of the bottom three only on goal difference but finished it two points clear of the drop zone.
Goals from Jaroslav Plasil and Cheick Diabate gave Bordeaux a 2-0 win that moved them above St Etienne and into sixth place in Ligue 1.
The home side went ahead through Plasil after 14 minutes and Diabate struck a late second to sink a St Etienne side whose European challenge has been hit by a run of two wins in 10 games at a crucial stage of the season.
Lens came from behind to salvage a 1-1 draw against hosts Auxerre but are now without a win in four in their bid to avoid relegation from Ligue 1. Willy Boly put Auxerre ahead at the Stade Abbe-Deschamps after only six minutes and the advantage stuck until Sebastien Roudet's equaliser 45 minutes later.
Caen remain in Ligue 1's bottom three after a 1-1 draw that does little for either them or visitors Toulouse. Romain Hamouma gave the home side the lead but Moussa Sissoko equalised from the penalty spot before half-time.
Valenciennes remain three points clear of the Ligue 1 relegation zone after frustrating ambitious Sochaux in a 1-1 draw at Stade Nungesser. Fouad Kadir gave the upwardly-mobile visitors an early lead but it was almost instantly cancelled out by David Ducourtioux's own goal.
Arles played for 80 minutes with ten men after Bobo Balde was sent off but the Ligue 1 basement boys still held Nancy to a goalless draw.