New signings Alex Oxlade-Chamberlain and Andre Santos scored in the first half Wednesday to help Arsenal overcome another defensive lapse in a 2-1 win over Olympiakos in the Champions League group stage.

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.

Arsenal then spent much of a nervy second half trying to neutralize the Greek club's attempts at a comeback. Vassilis Torossidis came close to equalizing in the 64th when his curling shot from outside the area hit the crossbar.

While the Gunners had looked set for a rout, in the end they were relieved to escape with a win -- having allowed a late equalizer in a 1-1 draw at Borussia Dortmund in their first game.

"We will be very happy with the three points," said Arsenal's assistant coach Pat Rice, who took charge in the dugout as manager Arsene Wenger was serving the second game of a two-match touchline ban. "It takes us closer to qualifying" for the knockout stage.

The win gives Arsenal four points from two games in Group F, while Olympiakos has zero.

"We're a bit disappointed," Olympiakos coach Ernesto Valverde said. "When you concede two goals that early, it affected us. It's a bitter taste, because we would have liked to have something more."

Despite having forwards Gervinho and Theo Walcott, midfielder Jack Wilshere and defender Laurent Koscielny ruled out with injuries, Wenger opted to rest captain Robin van Persie and start with Marouane Chamakh up front. Wenger also used midfielder Alex Song as a central defender next to Per Mertesacker in an attempt to shore up the defense.

Despite the makeshift lineup, Arsenal dominated early.

Oxlade-Chamberlain started in place of Walcott, and the similarly speedy winger only needed eight minutes to make his mark. The teenager ran onto a long ball from Song, chested it down and broke into the area with the help of a lucky bounce off a defender before slotting a shot between the legs of Olof Mellberg and to the right of goalkeeper Franco Costanzo.

It was Arsenal's 200th Champions League goal.

"It was a really good experience," Oxlade-Chamberlain said. "It's a big step up, but I want to play at the top level and I have plenty more time to play in the Champions League."

But the teenager's inexperience nearly cost Arsenal minutes later when he lost possession in midfield to gift Olympiakos a counterattack that resulted in a corner. The visitors nearly scored when the ensuing set piece led to a goalmouth scramble and Rafik Djebbour's close-range effort was cleared off the line by Mikel Arteta. Djebbour had a second chance from the rebound but sent his shot into the side netting.

Instead, Arsenal kept attacking and soon added a second goal.

Andre Santos made a raid down the left flank and crossed into the area for the onrushing Chamakh, only to have the ball cleared right back into his path before sending a low shot inside the near post.

Chamakh should have made it 3-0 in the 24th when Andrey Arshavin floated a perfect lob over the Olympiakos backline for the onrushing striker, who brought the ball down and had plenty of time to get off a shot, but still sent it wide.

Instead, Arsenal's defense broke down completely on another set piece -- like so many times before this season -- to let Olympiakos back into the game.

Ariel Ibagaza took a short corner to Kevin Mirallas before getting the ball back and broke unchallenged into the area before lofting a cross toward Fuster, who had wide-open space around him when he rose to power a header past a helpless Wojciech Szczesny.

"From a defender's point of view, it was really poor," Rice said.

The goal handed the momentum back to the Greek side, and Szczesny had to make saves from Djebbour and Torossidis in the next few minutes before Arsenal regained its composure.

The visitors continued to press after the break, but Torossidis' shot was the only attempt that was seriously threatening.

"Today we performed quite well in several parts of the game," Valverde said. "But when you play a team like Arsenal ... you have to take advantage of all your opportunities to score."

Aaron Ramsey came on for Oxlade-Chamberlain with 23 minutes left as Arsenal tried to shore up its midfield, and the Wales captain almost had an immediate impact as he intercepted a pass in midfield and released Arshavin, who dribbled into the area but had his shot easily parried by Costanzo.