كتب : Hatem Maher
Arsenal, with many youngsters in their squad, fought bravely but were undone by a goal from Leonardo two minutes after the restart.
The Brazilian winger found the net against the run of play when he slotted through the legs of keeper Lukasz Fabianski after latching onto a loose ball.
Olympiakos finished second in Group H on 10 points, three behind leaders Arsenal.
Belgium's Standard Liege earned a Europa League spot after a dramatic 1-1 draw with Dutch side AZ Alkmaar.
Jeremain Lens gave Alkmaar the lead three minutes before the break but an injury-time header from keeper Sinan Bolat helped Liege finish third on five points to book a place in Europe's secondary competition.
Alkmaar goal
Liege goal