Olympiakos' David Fuster, far right, scores against Arsenal during their Champions League group F match at the Emirates Stadium in London. (AP Photo/Kirsty Wigglesworth)
The 18-year-old Oxlade-Chamberlain netted just eight minutes into his Champions League debut and Andre Santos made it 2-0 in the 20th as the Gunners looked ready to finally put their disappointing start to the season behind them.
But Arsenal's much-maligned defense soon showed its frailties once again, letting David Fuster pull a goal back with a powerful header when left completely unmarked after a corner.
Vassilis Torossidis came close to equalizing in the 64th when his curling shot from outside the area hit the crossbar.
The win gives Arsenal four points from two games in Group F, while Olympiakos has zero.
