In a nail-biting climax, replacement Ross Byrne emerged as Leinster’s hero as his coolly-struck 79th-minute drop goal made it 20-18 and maintained the province’s unbeaten run in 2017 – nine wins and a draw.
The Ospreys did have a chance to overhaul the visitors in injury-time but Dan Biggar was off target with a penalty from the 10-metre line. The result means Leo Cullen’s men remain two points clear at the summit following Munster’s 10-7 victory over Glasgow Warriors.
Leinster recovered from a sluggish start in the Swansea sunshine – Sam Davies crossed for a fourth-minute try – as returning hooker Sean Cronin showed a great turn of pace to score under the posts in the 23rd minute.
Biggar’s second successful penalty saw the Ospreys retake the lead (11-10) for half-time, and the sides swapped converted tries from back rowers Dan Leavy and Justin Tipuric to set up a tense final quarter.
Captain Isa Nacewa missed a long range penalty in the 68th minute, but Leinster’s persistence was rewarded late on when Byrne lofted over the match-winning drop goal to start his birthday celebrations early – he turned 22 today.
Biggar, who led the hosts out on the occasion of his 200th cap, struck the right hand post with a difficult penalty in the opening minutes but the Ospreys hit the front soon after.
Rory Thornton supplied quick lineout ball, the half-backs and Josh Matavesi sucked in the Leinster defence and neat passing out to the right led to full-back Davies reaching over in the corner.
Biggar missed the touchline conversion but he had the Ospeys 8-0 up in as many minutes, punishing Josh van der Flier and Cronin for infringements in their own 22.
With Robbie Henshaw isolated after breaking onto a Jamison Gibson-Park kick through and full-back Nacewa making a couple of uncharacteristic errors, Leinster were struggling to make an impact.
The PRO12 leaders were turned over twice in contact and the Ospreys’ reliable lineout relieved some pressure after a classy touchfinder from Joey Carbery, deep in the 22.
However, Leinster were much improved in the second quarter. Nacewa threatened off scrum ball and when his side next attacked, excellent hands from Rory O’Loughlin and Henshaw released man-of-the-match Cronin to cut in off the right wing and he coasted past Biggar for a terrific try converted by Nacewa.
As Leinster attempted to kick on, a Gibson-Park knock-on spoiled a promising maul and then a terrific counter attack from Fergus McFadden came to an end when Rhys Ruddock’s attempted return pass to Gibson-Park did not go to hand.
As the interval approached, the energetic van der Flier got over a ruck ball to win a penalty which Nacewa turned into three points – only for Biggar to cancel them out with a crisp strike from distance in the final minute.
The second half was just six minutes’ old when Leavy rumbled over to the left of the posts, the replacement flanker twisting his way over after ground-gaining carries in the build-up from Zane Kirchner and Cronin.
Nacewa converted to open up a 17-11 lead and the Ospreys were thwarted in their immediate attempts to reply, the hard-working Ruddock securing a penalty-winning turnover in his own 22.
Cullen’s charges turned down a penalty kick from the left as they went for the jugular. However, Kirchner, who partnered Henshaw in the centre, was held up in a maul, and the Ospreys made them pay on the hour mark.
The increasingly influential Tipuric burrowed over just to the right of the posts after good initial work from an Ospreys maul and a strong burst from Rhys Webb. With Biggar off receiving treatment, Davies converted for an 18-17 scoreline.
It was nip and tuck thereafter, with Leinster shading the battle for possession and territory. Leavy’s speed and physicality at the breakdown earned a 68th minute penalty outside the 10-metre line which Nacewa nudged to the left and wide.
The visitors’ bench had the greater impact as the front row trio, including 150-cap man Mike Ross, kept up the pressure on the Ospreys scrum, and young scrum half Nick McCarthy provided snappy passes from ruck to ruck.
It looked like it might not be Leinster’s day when a loose pass from Henshaw went into touch – the fit-again Dave Kearney was his intended target – but the province regained possession and carried strongly through midfield. James Tracy, Ian Nagle and van der Flier, in particular, carried forcefully and gained yards to set up Byrne who lofted over his brilliant drop goal with 79 minutes on the clock.
There was still time for a response from the third-placed Ospreys. As they tried to set up their own drop goal opportunity, Byrne was penalised for a high tackle but Biggar pushed his place-kick to the right and wide, thus extending Leinster’s impressive recent record against Welsh sides – just one defeat in ten clashes with the regions.