A dismal first-half performance from the hosts had left them staring down the barrel of a costly defeat, but some ambitious changes from Alex Neil got them back in the game.
Even so, they had been labouring in search of a winner when substitute O’Nien raced onto a loose ball within fifteen minutes of his introduction.
After Elliot Embleton had cancelled out Ellis Harrison’s first-half goal a stunning strike from Jack Clarke added the gloss to what had for the most part been a difficult night for the Black Cats.
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In the end it was about the result and this was a crucial one.
Sunderland had come into the game seemingly growing in confidence on the back of two decent away performances, but from the off they were well short of the required pace.
Neil had an attacking line up, the loss of Nathan Broadhead offset by the selection of Patrick Roberts and Jack Clarke.
The hope was for a front-foot response but Sunderland gifted their struggling an opponents a way into the game, constantly losing the ball in dangerous areas.
Patterson had to make an early save to deny Danny Andrew, who had been allowed to cut inside from the left flank. Moments later a loose pass from Arbenit Xhemajli allowed Shayden Morris to break down the right flank, Jay Matete doing well to track back and snuff out the danger.
The first half an hour passed much that way, the movement and concentration desperately poor from the hosts.
They were given a welcome opportunity to reset when Patterson required treatment for a minor injury, Neil getting his side together to try and coax some sort of response.
Within a minute the Black Cats were behind and it was a goal that summed up the performance from the home side. Bailey Wright gifted away possession near his own box, the defence in no position to be able to recover. Harrison took the shot early, eaving Patterson exposed and unable to prevent the ball flying into the bottom corner.
If the increasingly and understandably frustrated home support hopes that might yield some kind of response from their players, then they were to be disappointed.
It continued to be Fleetwood who had the best openings, invariably on the break when Sunderland gifted it away.
Wright had to make a clearance just in front of his own line as the visitors continued to press forward, while a dangerous effort from Paddy Lane was just about cleared.
It wasn’t until the very final minutes of the half that the home side threatened, and the ease with which they began to find gaps only served to underline the frustration at what had come before.
Jack Clarke, whose delivery from wide had been wasteful, forced a strong stop Cairns when he broke towards goal from the byline, his low effort from a narrow angle cleared only bu the goalkeeper’s trailing leg. Sunderland recycled possession and when Winchester crossed for Embleton, he arguably should have done better in shooting straight at Cairns.
To his credit Roberts had looked lively when the hosts had been able to get the ball to him, and Neil looked to build on that by making two attacking half-time substitutions, Gooch and Defoe replacing Evans and Xhemajli.
The shape was unorthodox in the extreme, with Winchester and Cirkin taking a place either side of Wright in a back three.
Though it clearly left the hosts vulnerable to the counter attack, it did allow them to get more of their creative players involved in the middle of the pitch and the uptick in tempo and pressure was immediate.
The equaliser came within ten minutes, a scrappy but immensely valuable goal as Embleton managed to get it over the line and give his side a chance of taking three valuable points.
Sunderland were on top, but Patterson still had to be alert to make a good save to deny a dangerous Danny Andrew free kick that was heading for the top corner.
Fleetwood had settled and were beginning to find space, Patterson making another good save low to his left shortly afterwards. It was a big opening, and Garner ought to have done better from close range.
The momentum the hosts had been building had drifted, but Neil’s final throw of the dice proved to be crucial one.
O’Nien came on to replace Roberts, bringing some welcome dynamism into the middle of the pitch.
Stewart did well to turn the ball over with a big sliding challenge in the centre circle, and the hosts quickly moved it through the middle of the pitch. Defoe made a smart run into the box and though his effort was blocked, O’ien raced onto the loose ball across Cairns and into the far corner.
It had not been an easy watch, but Sunderland had the lead they so desperately needed.
Fleetwood served noticed of their threat when Patterson had to save well from Lane, but the hosts were given another major advantage thanks to another great piece of play from Stewart.
The hosts worked the ball into the channel and the Scot got the better of Jules, forcing the foul and a second yellow for the Fleetwood defender.
There was widespread dismay when the fourth official signaled eight minutes of stoppage time, but any fears of a late sting in the tail were allayed when Clarke drove in from the left flank and arrowed an outstanding long-range effort into the top corner.