Williams marks his 50th game in Europe with a 19th Heineken Cup try
Shane Williams and Tommy Bowe inspired the Ospreys to their first Heineken Cup win of the season as they overpowered Premiership leaders London Irish.
Bowe made an early breakthrough as Dan Biggar kicked four first-half penalties but the Ospreys dominance was reduced by Sailosi Tagicakibau's breakaway try.
But a sensational 80-yard chip and chase from Williams, before he touched down in the corner, sealed a fine win.
The Ospreys closed out a vital triumph without the sin-binned Jerry Collins.
The former New Zealand flanker was punished for going over the top but the Ospreys held on to extend their five-year unbeaten home record in Europe and deny Irish a bonus point in one of their finest Heineken Cup displays.
The Welsh region were desperate for their first victory in this 'Group of Death' and Bowe gave Scott Johnson the start the Ospreys coach demanded as he opened the scoring inside four minutes.
James Hook sprinted onto Dan Biggar's pass and his cute feint split covering defenders Seilala Mapusua and Daniel Bowden then his superb offload as he went to ground set unleashed Bowe.
And the Irish international went over against the Irish for his 18th score in his 38th European game.
But fly-half Biggar, criticised for his kicking display in last weekend's defeat in Toulon, again squandered the conversion chance early on.
Biggar's opposite number, however, was a 15-point hero in the Exiles' opening Hearken Cup triumph over two-time champions Munster last weekend.
And Ryan Lamb continued his confident display of kicking at the Liberty Stadium early on as he scored two penalties as the ill-disciplined Ospreys were punished after two off-side infringements.
Biggar, though, did find his range from inside his own half with a monster kick after Irish were penalised for off-side.
But it was the trademark twinkle toes of Williams that sparked this pool clash between the Magners League champions and English leaders into life.
Tommy Bowe scores his 18th try in his 38th Heineken Cup match
The Welsh wing danced past three defenders before splitting through the flat-footed visiting defence.
But his 30-yard dash to the line - including hurdling covering full-back Topsy Ojo - was halted by a Sailosi Tagicakibau and Kieran Roche sandwich just inside the 10 metre line.
The free-flowing Ospreys lit up a chilly night in Swansea as their star-studded back-line almost cut open the Exiles and after Biggar picked up the loose ball, his kick towards the try-line almost found Bowe but Ojo's last-ditch dive on the ball kept the dominant hosts out.
But despite the desperate defending, the Exiles' poor discipline was punished by a rejuvenated Biggar whose confidence with the boot had returned as the fly-half kicked three big penalties to edge Ospreys into a comfortable lead.
Toby Booth's side, however, were handed a route back into the game as Tagicakibau scored an interception try, Irish's second in successive European games, against the run of play.
The Samoan powerhouse broke clear after pinching Lee Byrne's pass to Richard Hibbard and sprinted 70 metres to score in the corner - despite Hook's best efforts to drag the 16st wing down
And as Lamb's conversion kicked Irish back within reach, Biggar dealt the Ospreys a blow as he was forced off injured.
But Williams ensured the Liberty Stadium crowd was not down for long as the Ospreys record try-scorer put his team firmly in the driving seat with a breathtaking score from just outside his own 10 metre line to mark his 50th Heineken Cup appearance.
The British and Irish Lion chipped over Irish's defensive line, caught the loose ball, skipped past Lamb and outpaced Tagicakibau to score his 105th try for club and country.
Hook, taking over the kicking duties from Biggar, kicked the conversion from an acute angle as he passed the 200-point milestone in Europe.
A composed 49-metre Hook penalty ensured the three-time Heineken Cup quarter-finalists would maintain their 13-month unbeaten record.
Lamb traded penalty blows but the Ospreys showed the maturity they perhaps lacked in defeat at Toulon last week to secure all of the points - even with 14 men for the final eight minutes - as they set their European standard.
Ospreys: Byrne, Bowe, Bishop, Hook, Williams, Biggar, M. Phillips, James, Hibbard, A. Jones, R. Jones, AW. Jones, Collins, Holah, Thomas. Replacements: N. Walker for Biggar (43), Bevington for James (67), Bennett for Hibbard (59), Mitchell for A. Jones (78), Gough for R. Jones (77), Tipuric for Holah (58).Not Used: Nutbrown, Owen.
Sin Bin: Collins (72).
London Irish: Ojo, Joseph, Mapusua, Bowden, Tagicakibau, Lamb, Hodgson, Dermody, Buckland, Rautenbach, Kennedy, Casey, Roche, Hala'ufia, Stowers. Replacements: Seveali'i for Joseph (71), Malone for Lamb (68), Allinson for Hodgson (77), Paice for Buckland (60), Corbisiero for Rautenbach (61), Garvey for Casey (60), Johnson for Hala'ufia (69). Not Used: Murphy.
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