The Ospreys inflicted a rare Ravenhill defeat on Ulster in Friday night's Celtic League encounter.
Ulster (6) 12
Pens: Steinmetz (4)
Ospreys (9) 20
Pens: Connor (4)
Drop Goals: Connor
Ulster failed to use the wind advantage in the first half and three Shaun Connor penalties gave the visitors a 9-6 lead at the break.
Paul Steinmetz's third penalty brought Ulster level before Jason Spice touched down in the corner on 59 minutes after getting on the end of his own kick.
An injury-time drop-goal from Connor sealed the victory for the Welsh team.
Steinmetz missed an eighth minute penalty but Connor made no mistake five minutes later to give the visitors the lead.
Steinmetz levelled after 25 minutes before Connor restored the Ospreys' advantage.
Ulster had plenty of possession but could find no way through a resolute Ospreys defence.
It was all-square again thanks to Steinmetz on 38 minutes but Connor fired over an injury-time penalty to give the Ospreys the edge at the break.
Ulster piled on the pressure at the start of the second half although a Steinmetz penalty on 56 minutes was all they had to show for their efforts.
Spice scored his unconverted try in a rare Ospreys break three minutes later, going over after Ulster flanker Campbell Feather failed to collect a kick into the corner.
Steinmetz reduced the gap to two points with his fourth penalty in the 62nd minute.
Ospreys continued to frustrate the Ulster attack and eadged further ahead with Connor's penalty on 79 minutes.
Ulster still had a chance but the drop-goal in stoppage time ended any hopes of a late comeback.
Ospreys coach Lyn Jones:
"We had 13 players out and I'm absolutely delighted with that result.
"We were honest and hard working to snuff their usual game out and force Ulster to play some rugby - they couldn't do that.
"This is the squad of guys who fronted up last year and I'm delighted for them.
"They deserve a few weeks off now, then we'll look forward to the three Welsh derbies in December."
Ulster: Cunningham, Topping, Maggs, Steinmetz, Maxwell, Wallace, Boss, J. Fitzpatrick, Brady, B. Young, Harrison, Caldwell, Feather, McMillan, Wilson.
Replacements: Larkin for Wallace (72), Moore for B. Young (25), Head for Caldwell (70), Ferris for Feather (63). Not Used: D. Fitzpatrick, Spee, Bell.
Ospreys: Cashmore, Mustoe, A. Bishop, L. Davies, Brew, Connor, Spice, James, B. Williams, Millward, Newman, Evans, Bater, Tandy, Beech.
Replacements: M. Jones for Connor (80), Pugh for Bater (69). Not Used: Hibbard, Griffiths, Bateman, Vaughton, Rees.
Referee: Ian Heard (Scotland)