The Dolphins' performance at Suncorp Stadium would have aged their 74-year-old coach Wayne Bennett after they almost gave up a 14-point lead for the third time in their past four games.
O'Sullivan, who came off the bench, kept a cool head when all around him were losing theirs to land the winner in the 83rd minute and take the Dolphins to 26 points and eighth on the ladder.
Bennett's men led 20-6 at halftime and the Warriors had Dylan Walker in the sin bin.
That 14-point advantage was restored when they led 26-12 in the 55th minute, before the Warriors scored three tries in nine minutes to level.
Centre Jake Averillo scored to get the Dolphins ahead 32-26.
Then came a pivotal moment when Dolphins No.6 Kodi Nikorima interfered with Warriors prop Mitch Barnett as he chased a Te Maire Martin grubber. Barnett got there first and knocked on. The Bunker awarded a penalty try and it was 32-32.
Dolphins winger Jamayne Isaako missed a field-goal attempt in regular time, as did Chanel Harris-Tavita for the Warriors.
Harris-Tavita hit one wide in golden point before O'Sullivan's heroics.
The Dolphins led Penrith and Gold Coast by 14 points in recent clashes only to lose, and they imploded again on Sunday.
The Warriors' season is all but over. On 21 points, they would need to win all their remaining games and rely on every other required result going their way to play finals.
Hooker Max Plath continued his stellar form, with centre Herbie Farnworth on fire on the back of Nikorima's guile.
The Dolphins got off to a strong start, as has been their habit, with veteran forward Felise Kaufusi planting a crafty grubber kick by Nikorima.
Farnworth raced 80m to score after hooker Plath swooped on an errant Warriors kick.
Warriors winger Edward Kosi out-jumped Jamayne Isaako after a Martin bomb to respond in style.
The electrifying speed of Dolphins centre Jake Averillo burnt opposite number Ali Leiataua to set up Nikorima for a cracker of a long-range try.
The Warriors controversially lost Walker to the sin bin for a high hit on prop Mark Nicholls on the cusp of halftime. Veteran Nicholls did not return.
Martin's kicking game kept the Warriors in it after the break, with another short kick leading to a try by Kurt Capewell, despite being a man down.
The Dolphins had the jitters for a short period, but crafty Nikorima sent Farnworth over for his second with a classic short ball to restore the 14-point margin.
The home side continued to invite the Warriors into it and Leiataua crashed through after a ludicrous Averillo offload went to ground.
Dolphins half Isaiya Katoa kicked the ball out on the full and Martin made him pay by chasing his own grubber to score, before giant forward Demitric Sifakula levelled it up at 26-26. Then all hell broke loose.
