Marseille suffered their first defeat under Jorge Sampaoli with a 3-0 loss at Nice on Saturday, while Bruno Genesio’s Rennes made it back-to-back wins by beating Metz.
OM had climbed into the European places after two straight victories, but slipped to sixth, below Lens on goal difference, after a poor performance at the Allianz Riviera.
Rennes are only a point behind in the race for the Europa Conference League spot after winning 3-1 at fellow European hopefuls Metz.
Teenager Khephren Thuram scored the opening goal on 34 minutes for Nice, before Amine Gouiri doubled the advantage with 16 minutes to play.
Dario Benedetto had a late strike for Marseille disallowed by VAR for offside, and their misery was compounded when Alexis Claude-Maurice added extra gloss to the scoreline for their Mediterranean-coast rivals in injury time.
Former Argentina and Chile boss Sampaoli, who took over from Andre Villas-Boas, had led his team to wins over Rennes and Brest.
Rennes backed up their first victory under ex-Lyon coach Genesio, against Strasbourg last weekend, with another impressive display.
Away forward Jeremy Doku netted in the 18th minute before Martin Terrier’s penalty seven minutes before half-time.
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The 18-year-old Doku was sent off early in the second half, but Serhou Guirassy wrapped up the points for Rennes late on. Papa Ndiaga Yade hit a 90th-minute consolation goal for Metz.
“Perhaps the success that had eluded this team for some time has returned,” said Genesio.