Shane Williams forces his way over for the Ospreys
The Ospreys moved into second place in the Magners League and made sure of Heineken Cup qualification thanks to Tuesday's bonus-point win at Ulster.
Shane Williams squeezed over for the opening try in the corner but Simon Danielli touched down for the province.
James Hook ran on to a Dan Bigger chip but Andrew Trimble's score put Ulster 18-12 ahead at the break.
Hook charged down Niall O'Connor's kick for his second try and former Ulster star Tommy Bowe got a try near the end.
The Ulster match was the postponed game in hand that the Ospreys had on all the sides above them except Leinster.
But that postponement could yet complicate their run-in, the Ospreys' late call-off of the original Ravenhill date the subject of a Magners League investigation next Monday that could, potentially, see the Liberty Stadium side docked points.
It was Ulster's fourth consecutive league defeat and they are in danger of missing out on next season's Heineken Cup should Connacht overhaul them over the course of the final few games.
The province got the opening points through a Niall O'Connor penalty but on 13 minutes the Ospreys scored their first try through Williams in the left corner after a Lee Byrne pass.
Ulster immediately hit back with another O'Connor penalty and then burst through to score, four minutes before the half-hour, as Isaac Boss, Ed O'Donoghue and Rory Best combined to see Danielli charge over the line.
The Ospreys needed to hit back and when a Biggar chip over the top was collected by Hook, he handed off Paddy Wallace to score. Biggar made no mistake with the extras and the Ospreys led again 12-11.
In first-half injury time, Trimble raced clear down the left wing to score with O'Connor this time adding the conversion to give Ulster the lead 18-12 at the interval.
Biggar narrowed that lead with an early second-half penalty and levelled the scores with another successful kick.
Ospreys delight at bouncing back
Home fly-half O'Connor handed Hook his second try after his chip was charged down in Ulster's 22, allowing Biggar to add the conversion to put the Ospreys 25-18 up.
Ulster came back again with two penalties from O'Connor, the second coming in the 66th minute after Ian Gough was binned for dragging down a driving maul, and Ulster then snatched the lead back 27-25.
But even with 14 men, the Ospreys managed to keep the scoreboard ticking over in their favour and Biggar kicked two more penalties to give his side a 31-27 lead.
Late on, a loose pass by Boss was hacked upfield by Adam Jones, allowing Bowe to touch the ball down as it rolled over the line.
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