He was also frustrated that the Reading players coped better than his own on a tricky pitch to play on.
"My main emotion is one of disappointment," BBC Sport quotes O'Neill as saying. "Conceding a goal so late in the match, coming off the back of a couple of decent chances we had, is frustrating. But we have to take it and look forward to the next match.
"Reading adapted to the conditions better than we did - the pitch was firm and a bit slippy on top. They started more brightly than we and got the goal which they deserved.
"We got back in through the penalty and from then on we were never out of the game. The game could have gone either way. To concede was very disappointing."
Sunderland are 13th in the Premier League after the result.