David Humphreys kicked a 40-metre drop goal just two minutes from time to seal a memorable win and claim the league title for the first time in Ulster's history. The Ospreys came close to wrecking their title aspirations when Gavin Henson converted Jason Spice's 75th-minute try for a 17-16 lead - with Leinster beating Edinburgh.
Humphreys took his total to 14 points on the night with his late drop goal, Matt McCullough scored their only try, that proved enough to seal the title. Justin Harrison lifted the trophy, that will now return to Ireland after two years in Wales, in the absence of captain Simon Best who missed this crucial game after breaking his ankle in last week's draw at the Scarlets.
The Ospreys drew first blood when Jonathan Vaughton scored inside eight minutes after second-rows Alun-Wyn Jones and Lyndon Bateman combined to open space out wide for the young wing. Humphreys hit back with a penalty before Ulster found their way over the line from close range. Justin Fitzpatrick drove to within inches of the line before McCullough leapt over to touch down.
Humphreys converted and added a penalty before half-time for a 13-10 advanatage but should have extended that three times. The 34-year-old dropped a pass with a clear field in front of him on the stroke of half-time and then missed straight-forward penalties on 52 and 60 minutes that threatened to cost Ulster the title.
The second of those came after Wales forward Thomas was sent o the sin bin for a professional foul after Stephen Ferris and Andrew Trimble threatened.
The Ospreys proved they would not give up their title without a fight when Lee Beech combined with scrum-half Spice to score before Humphreys stole it at the death.
Ospreys: G Henson; J Vaughton, S Parker, A Bishop, S Williams (S Terblanche, 77); S Connor (M Jones, 57), J Spice; P James (D Jones, 64), B Williams (capt, R Hibbard, 50), A Jones (A Millward, 57), A-W Jones (A Newman, 50), L Bateman, J Bater (R Pugh, 57), L Beach, J Thomas
Tries - J Vaughton, J Spice
Con - G Henson (2)
Pen - G Henson
Ulster: B Cunningham; T Bowe, A trimble, P Wallace, A Maxwell; D Humphreys, I Boss (K Campbell, 50); J Fitzpatrick, R Best, B Young, J Harrison (capt), M McCullough, N Best, R Wilson, S Ferris (N McMillan, 64)
Tries - M McCullough
Con - D Humphreys
Pen - D Humphreys (3)
Drop G - D Humphreys
Referee: David Changleng (Scotland)