London -Arsenal survived a chaotic defensive display to secure a 2-1 win over Olympiakos as Alex Oxlade-Chamberlain announced his arrival on the Champions League stage with a fine goal at the Emirates Stadium on Wednesday.
Gunners boss Arsene Wenger paid 10 million to sign Oxlade-Chamberlain from Southampton during the close-season even though the 18-year-old winger had only a year's experience in League One on his CV.
It is quite a jump from English football's third tier to Europe's elite club competition, but Oxlade-Chamberlain showed no signs of being overawed as he marked his Champions League debut with a cool strike early in the first half.
Andre Santos's first goal for Arsenal should have finished off Olympiakos but David Fuster got one back for the Greeks as the Gunners' defence went into meltdown.
Only the woodwork and some excellent stops from Wojciech Szczesny prevented Olympiakos from snatching a draw.
Wenger will know there is still plenty of room for improvement, but his side remain on course for the last 16 after taking four points from their first two Group F matches.
He had made the bold decision to leave captain Robin van Persie on the bench just days after the Dutch forward scored his 100th goal for the club against Bolton and then admitted he couldn't guarantee signing a new contract.
Beating Bolton had provided a much-needed boost to Arsenal's flagging morale after the club's worst start to a season for 58 years and Olympiakos, who had lost all nine of their previous visits to England, initially seemed to pose no danger to that revival.
Ernesto Valverde's team were so shaky in defence that they made Arsenal's notoriously fragile back-four look rock solid in comparison.
There was no need to fear these Greeks bearing gifts.
Arsenal took the lead in the eighth minute when Ivan Marcano's sloppy attempt to clear Alex Song's pass allowed Oxlade-Chamberlain to seize possession on the edge of the area and shoot low into the corner of Franco Costanzo's net.
Yet Olympiakos could have been level moments later when Arsenal failed to deal with a clever short corner and Rafik Djebbour stabbed goalwards, only for Mikel Arteta to come to the rescue with a tremendous goalline block.
Arsenal took advantage of that escape with a second goal in the 20th minute.
Tomas Rosicky sent Santos surging away down the left but when his cross was half-cleared back to the Brazilian, Marcano provided only the flimsiest barrier as Santos skipped past him before beating the flat-footed Costanzo at his near post.
At that point it seemed Wenger's side would win at a canter but Marouane Chamakh wasted a good chance to put the result beyond doubt.
However, Arsenal's defending remains their Achilles heel, especially with injuries to Laurent Koscielny and Thomas Vermaelen forcing Wenger to deploy midfielder Song at centre-back.
And the Gunners' rearguard was exposed as Olympiakos grabbed a lifeline in the 27th minute when Fuster was left completely unmarked to head Ariel Ibagaza's cross past Szczesny.
Wenger, watching from the stands as he served the last match of his touchline suspension, looked increasingly concerned as Olympiakos caused havoc on the counter-attack.
Djebbour's powerful drive was tipped over by Szczesny, while Kevin Mirallas forced the keeper into another good save.
Mirallas went close again soon after half-time, this time with a dipping half-volley, before Oxlade-Chamberlain squandered an opportunity to extend Arsenal's lead with a tame finish.
Olympiakos captain Vassilis Torossidis almost made him pay for that miss when he cut in from the right and rattled the Arsenal crossbar with a curling effort, but the Gunners held on.
Champions League results on Wednesday:
Valencia (Spain) 1, Chelsea (England) 1
Bayer Leverkusen (Germany) 2, Genk (Belgium) 0
Arsenal (England) 2, Olympiakos (Greece) 1
Marseille (France) 3, Borussia Dortmund (Germany) 0
Zenit St. Petersburg (Russia) 3, FC Porto (Portugal) 1
Shakhtar Donetsk (Ukraine) 1, APOEL (Cyprus) 1
BATE Borisov (Belarus) 0, Barcelona (Spain) 5
AC Milan (Italy) 2, Viktoria Plzen (Czech Republic) 0