Shane Williams weaved his magic with a spectacular try that grounded London Irish and gave the Ospreys' Heineken Cup campaign lift-off.
Williams' solo score helped inflict only a second defeat of the season on Aviva Premiership leaders Irish. The visitors could have few complaints as Williams' fellow Lions wing Tommy Bowe claimed an early touchdown before fly-half Dan Biggar kicked four penalties. Centre James Hook added a conversion and penalty, while London Irish mustered a Sailosi Tagicakibau interception try, plus 11 points from Ryan Lamb.
Ospreys were hungry from the start and delivered one of their finest European performances.
Irish will undoubtedly be in the mix at the qualifying business end in January, but it was a sobering night for them at the Liberty Stadium.
Conditions were perfect, and the opening exchanges did not disappoint as the Ospreys took just four minutes to open their account.
Irish struggled to clear the ball through a couple of poor defensive kicks and there was a ruthless nature about Ospreys' first attack as Hook sliced through the Irish defence before sending Bowe over.
Lamb settled Irish down, booting two penalties in quick succession for a one-point advantage midway through the opening quarter.
Biggar responded with a successful penalty. He later kicked three penalties during an eight-minute spell giving the Ospreys a deserved 17-6 interval advantage.
Irish knew they had to open the second-half scoring and the response came inside a minute when Tagicakibau intercepted Ospreys full-back Lee Byrne's pass for a 70-metre breakaway score.
Hook, clinging to the Samoan's shirt for the final 10 metres, tried his utmost to bring him down, but with Lamb adding a touchline conversion, Irish were firmly back in contention.
The Ospreys then lost a groggy Biggar -- replaced by Scotland wing Nikki Walker, with Hook moving to fly-half and Bowe switching into midfield. Ospreys needed a flash of inspiration to get them going again, and it arrived right on cue from a mesmeric Williams try.
It was a try good enough to win any game anywhere and when Hook booted a 49-metre penalty shortly afterwards, Irish looked down and out at 27-13 adrift.
Lamb's third successful penalty maintained chances of a losing bonus point but the home side were home and dry, their European campaign up and running in style.