Fletcher has five goals in just four Premier League starts for Sunderland since making a £12m move from Wolverhampton Wanderers, including the winning goal in Saturday's 1-0 success against Wigan Athletic.
O'Neill told the club's official website that he always believed Fletcher would start strongly despite eyebrows being raised over his transfer fee.
He said: "I could see his qualities when he played at Wolves and he's done everything I could possibly ask for.
"Of course you're delighted when someone you sign makes such an impact, but I have to say it hasn't really surprised me.
"He's got his goals, yes, but it's his all-round play and I think the crowd have taken to him. He's been brilliant for us."
Sunderland's unbeaten start to the season faces a stern examination in their next outing, a trip to reigning champions Manchester City on Saturday, October 6.