Stuttering Marseille suffered their second Ligue 1 defeat in four days on Friday, going down 1-0 at home to Nantes at the Stade Velodrome.
A solitary first-half strike from United States international Alejandro Bedoya gave Les Canaris their second consecutive win and lifted them above OM into fourth place in the table.
Marseille, who lost 1-0 at Lille on Tuesday, remain eight points adrift of the top three, with this latest setback further compromising their prospects of qualifying for next season's Champions League.
The gap from Elie Baup's side to Monaco in third could even increase with the principality side due to host struggling Ajaccio on Sunday.
A minute's applause was held before kick-off at the Velodrome in honour of former South African leader Nelson Mandela, who died on Thursday, while former Marseille striker Jean-Pierre Papin, one of the club's all-time great goalscorers who recently turned 50, was paraded to the crowd just prior to the start of the game.
How Marseille must wish they could call upon Papin at his peak now, as they toiled unsuccessfully to recover from the loss of Bedoya's early goal.
The former Rangers player converted a cutback from Filip Djordjevic in the 17th minute, with the ball dribbling into the net at Steve Mandanda's near post.
The hosts' best opening fell to substitute Saber Khalifa, who somehow blazed over the crossbar midway through the second half after excellent play from Andre-Pierre Gignac down the right wing.
Remy Riou in the Nantes goal also made two decisive interventions to deny Dimitri Payet in the second half, while Mandanda also saved well from Djordjevic at the other end.
But Les Canaris held on to inflict a damaging defeat on Marseille, whose under-pressure coach Baup oversaw Nantes's relegation from Ligue 1 during a spell in charge at the Stade de la Beaujoire in 2008-09.
"The defeat is a consequence of all the matches we have played since September, lots of intense games in which we have not taken points," said Baup, who was greeted with calls for his resignation by some supporters.
"Whether it be in the Champions League or against Lille or Nantes, these defeats weigh in our minds and on our bodies. And to that we need to add all the injuries. We must limit the damage between now and the winter break."
On Saturday, leaders Paris Saint-Germain will look to get back to winning ways when they host bottom side Sochaux at the Parc des Princes.
PSG fell just short of equalling the club record of 27 top-flight matches unbeaten when they lost 2-0 at Evian on Wednesday, their first defeat of any sort this season leaving them just a point clear of second-placed Lille and two ahead of Monaco.
Lille, who go to Bordeaux on Sunday, have gone 11 matches without losing or even conceding a goal, and their Nigerian goalkeeper Vincent Enyeama is within 141 minutes of the all-time French top-flight record of 1176 minutes unbeaten, held by former Bordeaux goalkeeper Gaetan Huard since 1993.
On Friday, Enyeama agreed a contract extension tying him to Lille until 2017.
Elsewhere this weekend, struggling 2012 champions Montpellier go to Toulouse on Sunday in their first game since the resignation of coach Jean Fernandez.