"I'm angry with the result. We should have had a deserved equaliser at the end of the game," O'Leary said after the 1-0 defeat at Villa Park.
"I've seen it on video and in my opinion it was not an offside when Kevin Phillips put the ball in the net.
"But this is typical of our season. We also gave away a bad goal.
"Darius Vassell's goal also summed up the season so far as I am concerned.
"I didn't think it was a very good game but we should not have lost."