A narrow win for Arsenal courtesy of two first-half goals but another defensive lapse meant the game finished 2-1, and was perhaps more nervy than was needed. Two goals in the opening twenty minutes from summer signings Alex Oxlade-Chamberlain and Andre Santos looked to have set the Gunners up for a routine three points but a lack of concentration at the back just before the half-hour gave the Greek visitor's hope.
Oxlade-Chamberlain became the youngest English goalscorer in the competition with a sole effort in the eighth minute, then Santos doubled the advantage reacting first to a rebound.
Olympiakos had rarely threatened but found themselves back in the tie on 27 minutes. The Gunners defence just went to sleep following a short corner and when the ball was eventually crossed by Ariel Ibagaza, David Fuster arrived unmarked to head home.
Vassilis Torosidis almost levelled in the second half but his left-foot effort beat the Gunners keeper but crashed back off the crossbar to safety, leaving Olympiacos having now lost all six of their away trips to England in the Champions League.
Referee: Carlos Velasco Carballo (ESP)
Venue: The Emirates
Yellow cards: Rosicky (ARS), Arteta (ARS), Fuster (OLY), Holebas (OLY), Djebbour (OLY)