Never before had the Gunners lost their first two group games and they now face a desperate scramble to maintain their 15-year sequence of reaching the competition’s knockout stage.
Defeat contributed to another chastening night for Premier League teams in Europe, with Chelsea's 2-1 loss at Porto making it five defeats from six group stage games for English sides.
Arsenal fell behind twice, to Felipe Pardo’s strike and Ospina’s own goal, but equalised through Theo Walcott and Alexis Sanchez, only to concede again to Alfred Finnbogason.
The hapless Ospina’s costly fumble proved the lasting image of a damaging evening, but a terse Wenger refused to scapegoat the Colombian or explain his decision.
“No keeper is mistake-free. It could have happened to Petr Cech as well. That’s part of it,” he said. “I do not have to sit here and give you any explanation for every decision I make. He [Cech] had a slight alert before the game at Leicester. I didn’t want to take a gamble. It’s not because of that we lost the game. It’s a farce. Look, I think I make the decisions and selections for the team and am responsible for it. And I know many things that maybe you don’t know and that you ignore. You cannot select the team by making a poll before the game and getting everybody’s opinion.”
Arsenal are used to making their lives difficult in Europe but appear to have started early this year, yet Wenger believes they can still finish second in Group F behind Bayern Munich.
“It leaves us in a bad position but we are still in it,” he added. “We have to of course think that we can deal with [Dinamo] Zagreb and Olympiacos even away from home and we have to make a result against Bayern at home. But we are not out of it.”
Olympiacos had lost their last six away games in Europe and all 12 of their previous visits to English opposition -- including three at Arsenal – but took a shock lead on 33 minutes when winger Pardo fired through a crowded box via Alex Oxlade-Chamberlain’s heel.
Arsenal hit back almost instantly, Sanchez slipping Walcott in to slot past butter-fingered goalkeeper Roberto.
Ospina outdid his opposite number just before half-time when he dropped a corner in the goalmouth and the fifth official ruled it had crossed the line.
Olympiacos’s creaking defence caved in on 65 minutes when Walcott chipped across goal for Sanchez to plant a downward header.
Parity lasted mere seconds, though, as Pardo took advantage of the visitors’ sloppy attempts to clear by crossing low for substitute Finnbogason to turn past Ospina from close range.