Fletcher started to repay the £12million fee Sunderland paid to secure his services from Wolves by bagging a brace either side of Wayne Routledge's strike to give the visitors a 2-1 interval lead. Michu powered in a header to level with his fourth goal in three games for Swansea.
O'Neill was delighted by Fletcher's contribution, and hopes he can be the goalscorer Sunderland have been lacking since the sale of Darren Bent. He said: "Darren Bent did very well here and Steven has come here to score goals. But he can also do well for us outside the penalty area, he has a lovely touch, but it was very pleasing for him to get off the mark."
He added: "With all the euphoria of Adam (Johnson) signing for us he was maybe in his shadow a bit, but he is a very good player."
The Wales left-back was hurt in a 16th-minute challenge with Gardner as the players collided after going for the ball, with Taylor's lower leg awkwardly buckling under him as the pair fell to the ground.
The home crowd were furious with Gardner over the injury, but O'Neill said: "I don't know why they thought that (it was a bad tackle by Gardner).
"He was trying to go past and the young lad has made a foul and it's very sad he has broken his ankle. But I think the crowd were laying into him and he was surprised by that and he came over to me to say he didn't do it.
"He was startled and he came over to tell me. We even had the free-kick, but Taylor is a very decent player and it is sad he is injured."
Gardner himself tweeted: "Hope Taylor is ok I never touched him but not a nice sight when a fellow professional goes off on a stretcher."