New manager O'Neill saw the Black Cats surrender the lead and squander a penalty before two-goal striker Fletcher earned Wolves a vital victory.
O'Neill, who will officially take charge as Steve Bruce's successor on Monday, watched from the directors' box as Kieran Richardson put the Black Cats ahead early in the second period.
But then Sebastian Larsson saw his penalty saved by Wolves keeper Wayne Hennessey before Fletcher struck twice in eight minutes.
Victory moved Wolves four points clear of the bottom three and relieved the pressure on manager Mick McCarthy.
It was Wolves' second win in 12 games but they have now taken seven points from their last three home matches.
Sunderland remain just a point above the relegation zone as their winless league run extended to five games.
The visitors were the first to settle and Stephane Sessegnon's volley was clung on to by Hennessey after Jody Craddock had headed away a Larsson centre.
Wolves threatened for the first time when Jamie O'Hara's volley ballooned up to Christophe Berra, whose header was held on the line by Keiren Westwood.
The home side appealed in vain for a penalty after Lee Cattermole tugged at Dave Edwards' shirt inside the box.
Hennessey went down to hold on to Larsson's long range attempt after a clearance from John O'Shea had dropped into his path.
Larsson made a vital tackle to halt the run of Stephen Hunt towards the Sunderland box.
It needed a superb save from Westwood to keep Sunderland on level terms after 39 minutes.
Matt Jarvis delivered a probing cross which found Hunt in space and his powerful close-range effort was parried away by Westwood when a goal seemed inevitable.
Lee Cattermole became the first player to be yellow carded a minute late for a challenge on Kevin Doyle and will now have to serve a one-game ban for incurring five bookings this season.
Stephen Ward came to Wolves' rescue three minutes before the interval when he cleared off the line from the impressive Sessegnon.
Bendtner went for goal himself at the start of the second period when the overlapping Larsson was in a better position and failed to hit the target with his rising shot.
But after 53 minutes a superb strike from Richardson put Sunderland in front after a quickfire counter-attack.
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.
Richardson then showed his defensive qualities with a vital clearance from Ward's low centre as the home side searched for an instant reply.
Ronald Zubar was booked for a foul on Bendtner before McCarthy made a double change with Doyle and Hunt replaced by Sylvan Ebanks-Blake and Adam Hamill.
Sunderland had the chance to double their lead when Craddock was adjudged to have brought down Larsson inside the box although replays suggested the initial contact may have been outside.
But Hennessey kept Wolves hopes alive by saving Larsson's low spot-kick away to his left.
It was a crucial save as within 30 seconds Fletcher had levelled for Wolves.
Jarvis sent over a curling centre and Fletcher's downward header beat the dive of Westwood.
Then Fletcher put Wolves ahead after 81 minutes when a cross from substitute Hammill was manoeuvred by O'Hara to the former Scotland international, whose low shot flew past Westwood.
Black Cats defenders appealed for a handball by O'Hara in the build-up - but replays appeared to show the officials got the decision right in letting the goal stand.
Bendtner was booked for a trip on Edwards as Sunderland were unable to rescue the game.