Ospreys fly-half Matthew Jones claimed a dramatic last-minute win with a long range penalty to deny visitors Leinster at the New Stadium on Sunday.
Ospreys: (14) 22:
Tries: A Bishop 2, B Williams.
Cons: M Jones 2.
Pen: M Jones.
Leinster: (9) 20:
Pens: Contepomi 5.
Andrew Bishop gave the Ospreys a dream start, scoring the first of his two tries after just 19 seconds.
Barry Williams also crossed for the Welsh, but Felipe Contepomi's boot kept Leinster in the hunt.
Wing Rob Kearney's try and Contepomi's fifth kick looked to have snatched the win, before Jones' last-gasp strike.
The Ospreys got off to a flyer when Bishop scored under the posts almost directly from the kick-off.
Kearney dawdled over a clearing kick and the young Ospreys centre charged down the ball and followed up to touch down, fly-half Jones converting.
Jones missed an easy penalty, but Contepomi - leading Leinster in Brian O'Driscoll's injury-enforced absence - showed how it should be done with a fine kick from wide.
When Leinster gave away a penalty the Ospreys spurned the kick and opted to go for touch and a five-metre lineout.
That adventure earned its reward as the Ospreys drove towards the Leinster line and skipper Williams popped out the back to score.
Jones again converted, but the fly-half then missed a difficult long-range penalty.
Two moments of indiscipline at rucks allowed Argentine Contepomi to slot late quick-fire penalties to cut the Ospreys' half-time lead to 14-9.
Lyndon Bateman continued the skulduggery after the restart, blatantly killing the ball in front of referee Rob Dickson and earning a yellow card.
Even worse for the Ospreys was Contepomi continuing his metronomic display with the boot, kicking his fourth penalty to cut the deficit to 14-12.
Leigh Davies came off the Ospreys bench and the former Wales centre had an immediate impact, mainly on the Leinster midfield with some shuddering tackles.
Jones missed another penalty, but with seven minutes left Bishop looked to have sealed an Ospreys win.
Bishop went over in the left corner for an unconverted try, the beneficiary of a superb pass out of contact by Davies and some swift hands.
But the Irish hit back almost immediately when full-back Girvan Dempsey broke free down the left wing and chipped ahead for Kearney to score.
Contepomi was agonisingly just wide with the conversion that would have tied the scores.
But when Jones spilled the ball and gave away a penalty as the clock ticked down, Contepomi stepped up to kick Leinster into the lead for the first time.
There was still time for one last twist, however, as Jones made perfect amends with his first penalty success of the night, a monster kick to make it 22-20 to the home side.
Ospreys coach Lyn Jones was relieved to have come away with a victory.
"It wasn't the best performance ever, but thanks to Matthew (Jones) - who played very well today - we came out on top," Jones said.
"It was difficult out there because the ball was very wet.
"But I'm just glad to win, it was a poor performance but there's plenty of games left for us to improve."
Ospreys: S Terblanche; R Mustoe, S Parker, A Bishop, A Brew; M Jones, J Spice; D Jones, B Williams (capt), A Millward, A Newman, L Bateman, L Beech, S Tandy, A Lloyd.
Replacements: L Davies, S Connor, R Wells, H Bennett, I Evans, P James, A Wyn-Jones.
Leinster: G Dempsey, B Burke, K Lewis, F Contepomi (capt), R Kearney, C Warner, B O'Meara; R McCormack, B Jackman, W Green, D Dillon, B Gissing, C Potts, K Gleeson, E Miller.
Replacements: E Byrne, D Blaney, S Crawford, J Heaslip, G Easterby, E Hickey, G Brown.
Referee: Rob Dickson (Scotland)