O'Neill was confirmed as Steve Bruce's successor on Saturday and the former Aston Villa and Celtic manager was in the stands at Molineux to get a first glimpse of his new players.
Eric Black was in temporary charge of Sunderland for this match so O'Neill's record hasn't been sullied just yet, but his stern expression as he watched the visitors throw away the lead against fellow strugglers showed he is well aware there is plenty of room for improvement on Wearside.
For a while it looked as though Sunderland would give O'Neill the perfect welcome present as Kieran Richardon fired the Black Cats ahead early in the second half.
But the turning point arrived when Sebastain Larsson's second half penalty, controversially awarded for a foul on the Swede, was saved by Wayne Hennessey in the 72nd minute.
Within 25 seconds Wolves had gone up the other end to equalise through Fletcher, who bagged the winner with a fine finish nine minutes from full-time.
That blow was made even more painful for O'Neill as replays showed Wolves midfielder Jamie O'Hara had used his arm to lay the ball into Fletcher's path before the goal.
Sunderland slipped below Wolves into 17th place in the Premier League table and O'Neill will officially take charge on Monday with his team only one point and one place above the relegation zone.
"I don't know if we were better than Sunderland or they were worse than us," Wolves boss Mick McCarthy said.
"It was two toiling, struggling teams who've been having difficult times. But my lads get going and you can see the pleasure it gives people around the place."
O'Neill had taken his place in the directors' box flanked by Steve Walford and Seamus McDonagh, who have respectively served as first-team coach and goalkeeping coach at his previous clubs.
Wolves threatened first when O'Hara's volley ballooned up to Christophe Berra, whose header was held on the line by Keiren Westwood.
It took a superb save from Westwood to keep Sunderland on level terms after 39 minutes.
Ryan Jarvis delivered a probing cross which found Stephen Hunt in space and his powerful close range effort was parried away by Westwood when a goal seemed inevitable.
Stephen Ward came to Wolves' rescue three minutes before the interval when he cleared off the line from Stephane Sessegnon.
Richardson put Sunderland in front when he finished a flowing counter-attack in emphatic fashion.
Nicklas Bendtner's flick found Sessegnon in space and he released the overlapping Richardson, whose rising shot flew past Hennessey inside the near post for his second goal of the season.
Sunderland had a golden chance to seal the win when Jody Craddock was adjudged to have brought down Larsson inside the box.
Replays suggested the contact - if any - may have been outside. But Hennessey kept Wolves' hopes alive by saving Larsson's low spot-kick away to his left.
Within seconds Fletcher had levelled for Wolves as his header from a Jarvis cross beat the dive of Westwood.
And Fletcher won it for Wolves in the 81st minute when a cross from Adam Hammill was nudged by O'Hara's arm to the Scot whose low shot flew past Westwood.
Source: AFP
Source: AFP