The Italians won 64-63 with Giorgi Shermadini netting the winning free-throw with just seven seconds left.
Bennet spread their scores around in a defensive battle, with their top scorer Maarten Leunen only scoring 12 points. Olympiacos on the other hand over-relied on Vassilis Spanoulis, who had 23 points.
Bennet now hold a 3-1 record in the group but their opponents have slumped to 1-3.
In the same group Fenerbahce moved to 2-2 with a 73-70 win at Bizkaia Bilbao with Croat Bojan Bogdanovic top-scoring with 19 despite starting on the bench.
In Group B Unicaja moved to 3-1 with a 99-85 victory over KK Zagreb (0-4) in Malaga.
There were also two games in Group B. Union Olimpija (1-3) crashed at home to Galatasaray (2-2) 79-70, while Russian side Unics Kazan (2-2) beat bottom side Asseco Prokom (0-4) 72-68 in Poland.
The top two in that group, Barcelona and Montepaschi Siena, meet in battle of undefeated teams on Thursday night.