Sean Holley’s men had been looking to leapfrog Heineken Cup holders Munster into first place in the standings but instead it is Leinster who are now the new league leaders.
The Dubliners came back from 5-0 down to win 13-8 in a low-scoring affair played in slippery conditions at the Liberty Stadium.
Gordon D’Arcy’s converted second-half try proved to be the match winner, before a second Felipe Contepomi penalty moved the visitors eight points clear.
The Ospreys did claim a losing bonus point thanks to a Dan Biggar drop goal with the last play of the match but the 2005 and 2007 Celtic League winners will be disappointed not to have taken a golden opportunity to further their 2009 title claims.
After Biggar had sent an eighth-minute penalty attempt wide of the uprights, the Ospreys were pressed into some desperate defending with 15 minutes on the clock.
Only impressive work from Cai Griffiths prevented Leinster from claiming the opening try of the match, as the prop beat Rocky Elsom to a loose ball over the Ospreys line after Shane Jennings had kicked through following an Ospreys knock on in midfield.
The Ospreys began to gain the upper hand in the set piece and it was from a lineout that they registered the first points of the night.
Former All Black No8 Filo Tiatia was stopped just short of the Leinster try line, with the Irish province appearing to have cleared the danger by gratefully clearing their lines courtesy of a fly-hack. However, the Ospreys kept the ball alive and a searing counter attack from Jonny Vaughton led to a try for outside centre Sonny Parker.
The Welsh international used his strength to burst through two tackles on his way to giving the Ospreys a 5-0 lead three minutes before half time.
Biggar struck the post with his attempted conversion to ensure the gap stayed at five points at the interval, but it was Leinster who began the second period the brighter of the two sides.
Their early second-half dominance was rewarded with 13 points without reply, with replacement half backs Chris Whittaker and Jonathan Sexton playing a major role in Leinster’s revival.
An early penalty from Contepomi reduced the gap to just two points before the Argentine added another to give Leinster a 6-5 lead following the sin binning of Tiatia for an infringement at a ruck deep in Ospreys territory.
A further seven points arrived while the hosts were down to 14 men, with 2005 Lions tourist and current Ireland international D’Arcy breaking through Gareth Owen’s tackle on his way to the crucial score.
Biggar’s dramatic drop goal at least salvaged a point for an Ospreys side shorn of numerous stars but it was Leinster who left for home the happier of the two teams.
