The Ospreys responded to Saturday's Heineken Cup quarter-final exit against Biarritz by claiming a valuable 38-27 bonus-point win over Ulster.
The win takes the Welsh region up to second place in the Magners League table ahead of Friday's clash at Leinster.
For Ulster, the defeat saw them slump to their fourth consecutive league defeat and leaves them vulnerable to missing out on next season's Heineken Cup should Connacht overhaul them over the course of the final few games.
Two tries from James Hook, in either half, along with tries from Shane Williams and former Ulster player Tommy Bowe saw the Ospreys boost their hopes of making the play-offs with Bowe's late try denying Ulster a losing bonus point.
Dan Biggar was also accurate with the boot and scored 18 of the Ospreys' total in a game in which the lead constantly changed hands before the Welsh side pulled away in the final minutes.
Ulster's scores came in the first half from Simon Danielli and Andrew Trimble but they were unable to punish Ospreys when Ian Gough was binned late in the second half.
Niall O'Connor made up for an opening penalty miss by nailing his first after 10 minutes but three minutes later Ospreys scored their first try through Shane Williams after a Lee Byrne flick pass.
Referee Neil Patterson consulted the TMO and the score was given though Dan Biggar's conversion bounced out off the upright.
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 and run round the behind the posts to narrow the angle for O'Connor, although his kick was adjudged to have missed by the TMO.
The Ospreys needed to hit back and when a Biggar chip over the top was collected by Hook, he handed off Wallace to score. Biggar made no mistake with the extras and now the Ospreys led again 12-11.
However, they were unable to build on their lead and, in first-half injury time, Trimble raced clear down the left wing with O'Connor this time adding the conversion to give Ulster the lead 18-12 at the turnaround.
Biggar then narrowed Ulster's lead with an early second-half penalty and levelled the scores with another successful kick after Ulster strayed offside.
O'Connor was then wide with a fairly straightforward effort and then gifted Hook his second try after his chip was easily charged down in Ulster's 22, allowing Biggar to add the conversion to put the Ospreys 25-18 to the good.
Again, though, Ulster came back 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.
Ulster again had a chance to hit back but lost possession on the Ospreys' line just as Gough returned to the action. You sensed it was a turning point and it proved to be with a loose pass from Boss being hacked upfield by Adam Jones, allowing Bowe to touch the ball down as it rolled over the line.
Tries: Danielli, Trimble
Pens: O'Connor 5
Tries: Williams, Hook, Bowe
Cons: Biggar 3
Pens: Biggar 4
Ulster: 15 Paddy Wallace, 14 Andrew Trimble, 13 Darren Cave, 12 Ian Whitten, 11 Simon Danielli, 10 Niall O'Connor, 9 Isaac Boss, 8 Robbie Diack, 7 Chris Henry (c), 6 Stephen Ferris, 5 Dan Tuohy, 4 Ed O'Donoghue, 3 BJ Botha, 2 Rory Best, 1 Bryan Young.
Replacements: 16 Andi Kyriacou, 17 Tom Court, 18 Ryan Caldwell, 19 David Pollock, 20 Paul Marshall, 21 Ian Humphreys, 22 Nevin Spence.
Ospreys: 15 Lee Byrne, 14 Tommy Bowe, 13 Sonny Parker, 12 James Hook, 11 Shane Williams, 10 Dan Biggar, 9 Mike Phillips, 8 Ryan Jones (c), 7 Marty Holah, 6 Jerry Collins, 5 Jonathan Thomas, 4 Ian Gough, 3 Adam Jones, 2 Huw Bennett, 1 Ryan Bevington.
Replacements: 16 Ed Shervington 17 Paul James 18 Alun Wyn Jones 19 Filo Tiatia 20 Jamie Nutbrown 21 Andrew Bishop 22 Nikki Walker.
Referee: Nigel Owens