The Ospreys sank to their second straight home defeat this season and their fifth consecutive Magners League loss to Leinster, who were indebted to 15 points from the boot of Jonathan Sexton.
These are worrying times for the Ospreys, who earned a bonus point but failed to show any real attacking flair despite the introduction of British & Irish Lions Lee Byrne, Shane Williams and Alun Wyn Jones.
Both sides swapped penalties during the opening skirmishes. Leinster opened the scoring with a penalty through Sexton after Marty Holah failed to release at a ruck and three minutes later the Welsh side were back on level terms through a James Hook kick following an offside decision.
The Ospreys got themselves in some trouble in their own 22 but got a penalty decision in their favour at a scrum in the shadow of their posts.
But crossing in their own half saw Leinster get back into the lead with a second Sexton penalty on 15 minutes.
The Ospreys turned defence into attack and won a penalty as Shaun Berne failed to release at a ruck.
But Hook failed to convert his attempt from 35 metres out on the right.
A golden chance of an Ospreys try went begging when Jamie Nutbrown made a break from a scrum on the 22 but could not find wing Tommy Bowe with his long pass out to the right.
But the Ospreys pressure continued and built into a try scored by Jerry Collins, his first for the region, as the home side mauled a line-out from five metres out over the line.
At 8-6 down Leinster had two chances to regain the lead but Sexton missed penalty goal chances on 30 and 35 minutes.
The Ospreys looked in real trouble just before half-time when they had Collins (violent play) and Duncan Jones (professional foul) sent to the sin-bin in the space of three minutes.
However, they managed to hang on to their two-point advantage until the break.
As in the first half Sexton and Hook swapped penalties, but it was Leinster who built up a head of steam in the final quarter despite the Ospreys bringing on Mike Phillips, Williams and Alun Wyn Jones.
A mistake by Williams allowed Leinster to get in an attacking position from where full-back Isa Nacewa was able to slot over a drop-goal.
And two further Sexton penalties on 75 and 80 minutes gave Leinster enough of a cushion to survive until the final whistle.