The Ospreys grounded Gloucester in emphatic fashion to keep alive their Heineken Cup quarter-final hopes as former All Blacks star Justin Marshall ran the show.

James Hook kicked 22 points, while fellow Wales star Gavin Henson - on his first appearance since December 2 - effortlessly eased through the gears.

But both players were eclipsed by master scrum-half tactician Marshall as the Ospreys made light work of treacherous conditions to inflict a Pool Two drubbing on the Guinness Premiership leaders.

The west country club will still secure a last-eight place as group winners if they beat Ulster at Kingsholm on Sunday week, but top seeding could elude them if Pool One leaders London Irish continue their free-scoring ways in Treviso next Saturday.

The Ospreys, though, should clinch one of two best runners-up spots by defeating French hosts Bourgoin after giving themselves a lifeline at Gloucester's expense.

At times, Gloucester were an indisciplined shambles.

They had three forwards sin-binned - flanker Peter Buxton, plus substitutes Luke Narraway and Olivier Azam - and played almost eight minutes of the second period reduced to 13 men.

It was comfortably their worst performance of the season after the Ospreys outclassed and outmuscled them in every critical area.

Wing Shane Williams and substitute hooker Richard Hibbard scored tries for the Ospreys, while Rory Lawson and Ryan Lamb claimed Gloucester touchdowns, with Lamb booting one penalty and Chris Paterson slotting a conversion.

Watched by an 18,000 crowd, Henson made his first start since suffering a broken hand during the Ospreys' EDF Energy Cup victory over Harlequins six weeks ago, returning less than 48 hours before new coach Warren Gatland names Wales' Six Nations squad.

Gloucester were again without England Six Nations squad member Lesley Vainikolo - he is mourning the death of his father in New Zealand - so Leon Lloyd continued to deputise, but Iain Balshaw, James Simpson-Daniel and Gareth Delve all returned from injury.

Rain lashed the Liberty Stadium, starting some 90 minutes before kick-off and maybe reminding Gloucester of their Premiership defeat in a deluge at Bath eight days ago.

And it quickly became a nightmare for Gloucester as they fell 10-0 behind in as many minutes.

Hook's opening penalty was followed by a piece of Henson brilliance as he gathered his own kick one-handed on the bounce under pressure from three Gloucester defenders.

Henson almost made it to the line in thrilling fashion, but Ospreys moved possession quickly from the resulting ruck and an unmarked Williams skipped over.

Hook added the extras, then slotted another penalty to leave Gloucester - who lost lock Alex Brown to an early injury - in dire need of Lamb's 26th-minute three-pointer.

It was a top-class performance by the Ospreys, who further profited from Gloucester's growing sense of frustration when Hook completed his penalty hat-trick.

Gloucester's forwards needlessly pulled down a maul, then Hook's task was made even easier by Lamb's show of dissent to referee Alan Lewis that resulted in the visitors being marched back 10 metres.

Lewis' patience was pushed to the limit just eight minutes later, and this time he sin-binned Buxton for blatant offside that illegally halted promising Ospreys ruck ball.

Hook again kicked the three points, leaving disorganised Gloucester needing all kinds of half-time inspiration from their head coach Dean Ryan as they trooped off 19-3 adrift.

Ryan's response was to make two substitutions within seven minutes of the restart, sending on Azam and utility back Paterson.

Unlike against Bath last week though, Paterson replaced wing Lloyd rather than struggling fly-half Lamb, and it made little difference to the overall picture as Hook slotted a fifth penalty.

Gloucester's wet-weather rugby was again way below par, despite Lawson giving them a glimmer of hope when he sprinted 40 metres unopposed to touch down just 90 seconds after replacing Gareth Cooper.

Paterson converted as a gentle reminder to the Ospreys the game was not yet over but another Hook penalty eased any nerves before Gloucester imploded.

Narraway was yellow-carded, then the reckless Azam followed suit for a pointless lunge on Ospreys flanker Marty Holah.

In the meantime, Marshall was replaced to a standing ovation as Ospreys supporters began celebrating when Hibbard completed Gloucester's misery.