The Greek side went ahead in the first half through Mady Camara, only for Dimitri Payet to score two penalties after the break to give OM their first win and their first goals in the competition this season.
OM, who ended an embarrassing 13-match losing streak in the Champions League, are bottom of the group on three points, behind Olympiakos by virtue of the head-to-head rule.
Olympiakos host Porto...