"The referee has no-one in his line of sight," O'Neill told BBC Sport. "Corry's arm isn't in an unnatural position, it's by his side. The ball hits him on the back more than anything. I thought the referee had blown for a foul or an offside. Nobody had claimed for it.
"I'm staggered by the decision, staggered by the yellow card. It's such a defining moment in the match. The opening tackle by Fabian Schar was borderline. I thought it was a red card. The referee hasn't done us any favours.
"We have to forget about it. I thought the players' reaction to it was very good. We played much better in the second half, the game was even. We are still in the tie. Maybe a referee will give us a decision in the second leg.
"I'll have to pick the players up. There's anger in the dressing room. We're going to have to find a way to get a goal back. Stuart Dallas's injury is a blow. I thought the players who came on did well. We might look to freshen the team up on Sunday. We have to channel the anger."
Northern Ireland are aiming to take part in back-to-back major tournaments for the first time, having played at Euro 2016 in France.