O'Connor kept his cool to cancel out Massimo Luongo's 70th minute-opener - his thumping finish with 90 seconds to go setting the tie up perfectly for the second leg at Griffin Park on Monday.
It was rough luck for Tottenham loanee Luongo, who went from hero to villain as he made the crucial foul at the end of Brentford's best move of the night.
And given the penalty furore that surrounded the Bees' 1-0 defeat at Doncaster last Saturday - they missed one to lose an automatic promotion spot - O'Connor's cool head was even more impressive.
"Kevin is excellent," said Rosler. "He had not played for a long time but stepped up to the plate. It was a difficult situation for him as he was not playing many matches, but in all the games he looks very solid and does a great job.
"Obviously, in the end, he used all his experience and skills to convert the penalty and I'm very happy for him."
The German manager added: "The tie is wide open and I said before the game, we must make sure it was.
"To get a draw after all the circumstances of last week was a very good result.
"It was a difficult game with not much between the two teams. In the first half they pushed us back but we had the best chance.
"After seeing our games this season against Swindon, every time we've played them in a 4-4-2 and we've played well, that was the main reason we started that way, to match them up."