Scottish champions Celtic, under fire after a timid 3-0 derby defeat at Ibrox, will look to return to winning ways when they host Hungarian side Ferencvaros in their Europa League opener on Thursday.
They have a chance for immediate revenge on Rangers in a rematch in the Scottish Premiership at Parkhead on Sunday.
"Every game that Celtic lose, there's a crisis, and of course even bigger if you lose to Rangers, so we've done enough talking about it," O'Neill said at a pre-match press conference on Wednesday.
"Our players are disappointed with the performance, we could have done better, I think we accept that, and we just apply ourselves now again for the league fixtures."
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O'Neill, who in his second full-time spell as Celtic manager, added: "We were disappointed ourselves, we disappointed the fans and now we just have to rise to the occasion.
"We've got two matches in which to do so before the break. Two big games for us, Ferencvaros will not be easy, but we've done it before, we'll get ourselves right."
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Five of O'Neill's starting line-up were playing in an Old Firm match for the first time on Sunday and the 74-year-old said: "I think it was an eye opener to some of the newer lads.
"We've got to find out about players, about whether they're capable of dealing with these games.
"Hopefully, over the course of time, they will be."