The Colombian goalkeeper inexplicably dropped a fairly tame corner from Kostas Fortounis in the 40th minute of his side's Champions League defeat at the hands of Olympiakos at the Emirates Stadium.
Ospina, who was playing in place of Petr Cech, fumbled the ball over the line to give the Greek side a 2-1 lead in their eventual 3-2 defeat but Wenger was not looking to point the blame in his direction.
Regarding the decision to select Ospina, Wenger said: "That is a simple thing.
Wenger added that Cech's absence was not the reason the Gunners were beaten before once again defending his selection process.
"He had a slight alert before the game at Leicester and I did not want to take take a gamble but it is not because of that we lost the game," he added of Cech.
"I make the decisions and selections for the team and I'm responsible for it. I know many things that you don't know and that you ignore. You cannot select a team by using a poll and getting everyone's opinion before the game."