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. Matthew Morgan converted Bevington's try and added three penalties for the away side.
Ulster had to be content with the losing bonus point from their first home league defeat since succumbing to Leinster in April 2012 and they may also have to do without star player Ruan Pienaar, who kicked over three penalties before he was taken off in the second half with a shoulder injury. Stuart Olding added Ulster's final penalty.
Leinster completed a comfortable double at Cardiff Arms Park to see off Cardiff Blues 26-11 and consolidate their position in the play-off places.
The Blues had been enjoying something of a revival at home, with three successive wins in three different competitions, but were outmuscled up front and out-thought at the back in a disastrous first half, meaning even a gallant fightback after the break was never going to be enough.
Both sides showed plenty of enthusiasm in a lively opening before Leinster took the lead with the first of four Ian Madigan penalties.
Isaac Boss and Jordi Murphy crossed over for the away side as Madigan added the extras to take his points tally to 16. Jason Tovey scored a second-half try for the Blues and Rhys Patchell kicked over two penalties.
Fly-half Owen Williams kicked 20 points as the Scarlets sealed a 25-15 victory over Connacht at Parc y Scarlets.
Lock Joe Snyman scored the Scarlets’ only try - his first try for the region.
But the Welsh side found themselves out-scored two tries to one by Connacht who were indebted to scores from wing Daniel Poolman and Ronan Loughney at either end of the contest.
And Newport Gwent Dragons came from behind to score 17 unanswered points and secure a desperately-needed 23-14 win over Italian outfit Benetton Treviso at Rodney Parade.
Treviso looked to be in the driving seat after they led 14-6 at the break courtesy of tries from centre Doppies La Grange and replacement back row Filippo Giusti.
But the Dragons, whose only points in the first period came from the boots of Steffan Jones and wing Tom Prydie, turned the match on its head as both men added tries and Prydie kicked seven extras.