The Ospreys battled hard to secure a narrow win in a tight encounter at the Liberty Stadium.
Dan Biggar kicked fourteen points and the Ospreys also scored a penalty try, while man of the match Isa Nacewa scored ten points, including a try, with Leinster’s other try coming from wing Andy Conway.
It was the Ospreys who made the early running on a cold evening at the Liberty Stadium, with Barry Davies and Ashley Beck both showing up with positive runs at the heart of the Leinster defence in the opening minutes.
They were rewarded with the first points of the game through the boot of Biggar, the fly half having the simplest of tasks to slot over a penalty from right in front of the posts following a pick and go from Rhys Webb, the Leinster tackler penalised for not rolling away.
The Ospreys continued to hold the upper hand, but with a quarter of an hour gone, a series of quickfire penalties against them for not rolling away allowed Leinster to move upfield into kicking distance, INacewa making no mistake after Ian Evans was the offender.
Referee David Wilkinson had given a strong warning to the Ospreys, and when Duncan Jones was penalised for coming in from the side shortly after, Mr Wilkinson was left with no option than to show him a yellow card.
With the extra man advantage Leinster immediately looked to stretch the Ospreys defence, first moving it right and than back across to the left hand side, where Nacewa did well to shrug off the combined efforts of Biggar and Gareth Owen to offload to Conway who scored despite the attention of Barry Davies, Nacewa converting.
The response from the Ospreys was a positive one, with Owen being held up just short as he looked to force his way over before Richard Fussell failed to find Sonny Parker on his outside as they looked to work an overlap on the left.
Leinster again threatened, Fergus McFadden and Dominic Ryan both making good yardage, but the Ospreys defence was able to prevent ant further damage before they were restored to 15 with the return of Jones just before the 30-minute mark.
Biggar then slotted over his second penalty of the evening after Ian Madigan was pinged for not rolling away, taking the Ospreys to within four points of Leinster. The deficit was just a point minutes later, a massive Ospreys shove in the scrum leading to a penalty and once again Biggar made no mistake.
Mr Wilkinson was incurring the wrath of the home support after a series of Leinster offences, and shortly before the break finally issued a team warning to skipper Leo Cullen after Conway was caught playing it on the ground.
HALF TIME: OSPREYS 9 LEINSTER 10
The second half began in the same manner, the Ospreys on the attack and Leinster offending, allowing Biggar to make it four from four just 90 seconds in to put his side back in front.
Leinster hit straight back, Fussell doing enough to stop Conway winning the chase to ground a clever grubber to the corner from Nacewa, but as the ball spun loose, the fullback was following up and able to touchdown, the TMO swiftly confirming a try before Nacewa failed with the conversion attempt from wide right on the touchline.
There was then a lengthy delay while Leinster’s Conway received treatment for a neck injury, both sides having to make huge efforts to stay warm on such a cold night, Conway eventually being stretchered off after eight minutes of treatment.
Lock Ed O’Donoghue was sent to the sinbin shortly after the action got back underway for blatantly going off his feet at a ruck, right under the referee’s nose. It could have been a second yellow just a minute or so later after Jason Harris-Wright spear tackled Marty Holah but Mr Wilkinson opted for a talking to. However, that second yellow did appear, Ian Madigan joining O’Donoghue in the bin for killing the ball as the Ospreys looked to force their way over from the resulting penalty lineout.
With the Leinster pack a man down, the Ospreys opted for a five metre scrum. Numerous times the visitors went down as they were driven backwards, until the inevitable penalty try arrived, Biggar adding the extras to make it 19-15.
Nacewa had the chance to cut the gap to a point with a penalty from 40 metres out after Tom Isaacs was in front of the kicker as Owen looked to clear, but his penalty attempt fell just short.
As the game reached the hour mark Leinster piled the pressure on close to the Ospreys line but they were unable to find a way through, with Holah doing some excellent work at the breakdown to secure turnover ball to relieve the pressure.
Having enjoyed a good night with the boot, Biggar then attempted a penalty from just inside the Ospreys half but his effort lacked length and direction.
At the other end Fussell showed good awareness to gather a difficult Dave Kearney up and under as Leinster looked to pounce on any mistake.
It was delicately poised, and again Leinster came at the Ospreys, going through phase after phase to inch their way forward, but time after time they came up against a black wall.
With the Ospreys looking to play the clock down there was one last, late chance for Leinster after they won a penalty with less than 90 seconds remaining. The ball was kicked to the corner and the visitors looked to force their way over from the resulting line-out, but again, the Ospreys defence held firm. With time up, Richard Hibbard was able to win an important turnover, allowing Biggar to clear to touch, the Ospreys securing a valuable win.