It was the Principality side's fourth win in five fixtures, with their only blip so far coming in the shape of a defeat to defending champions Lyon.
The tight contest was settled on 38 minutes when Sebastien Squillaci headed in Jerome Rothen's cross.
Marseille kept pace with Monaco at the top of the table after a similarly narrow win over Ajaccio.
New signing Camel Meriem proved to be l'OM's hero as he poached the only goal of the game on 64 minutes.
Nice's unbeaten start to the season came to an end at Lens as they were defeated courtesy of a Dagui Bakari (pictured) goal.
Bakari netted the all-important strike on 71 minutes when he darted ahead of Eric Roy at the near post to fire beyond Nice goalkeeper Damien Gregorini.
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