Leaders Ulster fell to only their second PRO12 League defeat of the season when their weakened side was outplayed by an Ospreys outfit that, despite missing even more squad members than their hosts, outplayed the Ravenhill squad.
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In a closely-contested game that was nip and tuck until the later stages, it took a breakaway try by Ospreys prop Ryan Bevington to seal the four points for the visitors who at that stage were reduced to 14 men after Morgan Allen had been sin-binned moments earlier.
Ulster had to be content with the losing bonus point from their first PRO12 home defeat since succumbing to Leinster in April 2012 and they may also have to do without star player Ruan Pienaar who was taken off in the second half with a shoulder injury.
Pienaar had opened the scoring with a long-range penalty after three minutes which was evened out six minutes later by Matthew Morgan after Paddy Wallace was pinged at the breakdown.
Morgan was wide with a snap drop goal attempt after skipper Kahn Fotuali'i ran a close in penalty but then pretty much from the restart, Ulster carried just inside Ospreys territory and with the Welsh going off their feet Pienaar nailed the even longer-range penalty to put the home side back in the lead after 12 minutes.
It did not last though and when Mike McComish attempted to rumble forward off a lineout, he was penalised for holding on and the soft 20th minute penalty was slotted by Morgan to tie the scores again.
It then took a big hit from Michael Allen on Richard Fussell to snuff out another promising Ospreys attack, and then Scott Baldwin was penalised for holding on just short of the Ulster line, before Ulster managed to get on the front foot.
The result was Pienaar's third penalty just after the half hour when the Ospreys were again pinged at the breakdown following a Wallace break.
But it was the Ospreys who finished the half the stronger when they attacked out of their 22 following a lineout turnover only for Fussell to knock on with Ben John waiting for the pass outside him and Ulster trooped off 9-6 in front.
On the resumption the Ospreys quickly took control and were bolstered by the return to action of Alun Wyn Jones who was brought on off the bench for his first game time since November.
After Fotuali'i was wide with a rather rash drop goal attempt under Ulster's posts, Ali Birch was pinged at a ruck and Morgan made it 9-9 after 53 minutes.
Things then got considerably worse for Ulster with Pienaar then leaving the field with what appeared to be a shoulder injury but Ulster responded with their best period of possession sparked by Paul Marshall's arrival and some great carries from Robbie Diack and McComish.
Olding landed the inevitable penalty which saw Morgan Allen shown yellow three minutes after the hour to make it 12-9.
But then after Marshall had sparked a huge Ulster surge, the Ospreys countered from their own 22 with Jonathan Spratt linking with Fotuali'i, the scrum half taking out three defenders with his infield pass to an on onrushing Bevington who ran in from 30-odd metres.
Morgan converted and the Ospreys led 16-12 with only 14 men on the park and indeed did not concede again before Allen's return and the Welsh side finished the game on the front foot.