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The RDS was thronged to see two of the league's top teams put their early-season unbeaten records on the line and it was a defensively-sound Leinster who came up trumps with Contepomi and Nacewa kicking 14 points between them.
However, come May, Leinster could look back and rue the last minute of this game when they allowed Nikki Walker to barge through for a try which James Hook brilliantly converted from the left touchline, to salvage a bonus point for the Ospreys.
Sean Holley's side, showing 12 changes from last week's win over Glasgow, lost Sonny Parker to a training ground knock and had to draft David Bishop into their midfield to partner his brother Andrew.
Leinster lined out as selected with New Zealander Nacewa slotting in at number 10 for the first time and team captain Leo Cullen returning from shoulder surgery.
The Ospreys made much the better start. Scrum-half Rhys Webb sniped around the fringes and teed up Hook for a fourth-minute drop-goal attempt which he miscued.
Contepomi, playing again at inside centre, was a ball of energy and his interception and dinked kick out to the right was well taken by the advancing Nacewa, as Leinster edged into the visitors' 22.
Both sides pressed and probed but it took a 20th-minute drop goal from Hook to open the scoring after a concerted build-up from the Welsh pack.
Lee Byrne and Tommy Bowe threatened out wide for the Ospreys but it was Leinster's in-form winger Luke Fitzgerald who broke the try deadlock after 24 minutes.
Brian O'Driscoll's flat pass out to the left sprung Contepomi into space and with Bowe and his team-mates left chasing, three deft passes between the Argentinian and Fitzgerald put the 21-year-old over for his fourth try of the campaign.
Contepomi comfortably converted but the Ospreys stormed back before the break. Hook kicked the first of two penalty attempts to cut Leinster's lead to 7-6 at half-time.
The Ospreys upped the ante on the resumption, controlling play for long spells but never really stretching the Leinster rearguard.
Prop Paul James came off the bench to make a record 102nd appearance for the Welsh region, as the game hung in the balance.
But Leinster, coming to the fore in the final quarter, showed an enviable ability to score off two or three phases.
Contepomi landed a morale-boosting penalty to ruin the Ospreys' hard work. And while the visitors' handling let them down, Leinster began to click within sight of the finishing line.
Helped by a sterling lineout effort by man-of-the-match Bernard Jackman, Leinster gained another kickable penalty which Contepomi duly converted.
Cian Healy then had a try ruled out for a knock on, yet Leinster's late flourish was suitably crowned by two priceless drop goals from Nacewa, both from close range.
At 19-6 ahead, Leinster looked to have done enough to send the Ospreys home with no reward, but up popped monster winger Walker to charge over in the left corner after a 50-metre solo burst.
Hook's conversion had added meaning as the bonus point moves Holley's men to the top of the table, with Leinster third and with a game in hand.