The Scottish international was doubtful for the game after suffering an ankle injury last week, but made a speedy recovery in time for kick-off.
He managed to get on the score-sheet as the Black Cats rose out of the relegation zone, and O’Neill was delighted with his striker’s attitude:
“Fletcher had done absolutely no work whatsoever. When you were talking to me yesterday, I genuinely wasn’t sure that he could play in the game.” He told SkySports.
“He came out this morning just to do a bit of a loosener and thought he wanted to give it a go.
“I personally thought if he could see it to half-time and just slightly into the second half, he would do great for us.”He has been terrific for us, really terrific. You just feel that he is likely to get you a goal in there.
“Danny Rose did very, very well, but when it came in, he turned it into a goal for us and it was great.”
James McClean gave Sunderland the lead after just three minutes, before Fletcher flicked home with half-an-hour gone to double the lead. Stephane Sessegnon completed the scoring late on with a calm finish.