It means the Ospreys must win in Bourgoin next weekend to snatch one of the two best runners-up spots in the last eight - assuming Gloucester complete the double over Ulster at home to win the Pool.
On a rain soaked night the Welsh region were never behind and built up a fine head of steam both on the pitch and on the scoreboard in the first half.
Hook's unerring boot provided four penalties in the first half and he also converted the game's first try from Shane Williams. That score emanated from some unusual hesitancy in the Gloucester defence after Gavin Henson had hoisted an up 'n' under.
Having seen the ball bounce, Henson jumped to retrieve the ball before running the length of the 22 to get within inches of scoring himself. The ball was successfully recycled and Williams was put over in the right hand corner.
Gloucester lost the first of three players to the sin bin just before half time when Peter Buxton was given a 10 minute rest by Irish referee Alan Lewis for taking out a player at a ruck. Two more were to follow late in the second half - replacement Luke Narraway and Olivier Azam for bad language and a high tackle respectively.
Hook extended the home side's lead with his fifth penalty ten minutes after the break, but then Gloucester grabbed a sucker try when replacement scrum half Rory Lawson peeled around a line-out before racing 25 metres to score. Chris Paterson added the conversion and suddenly the gap was only 12 points.
Hook's sixth penalty increased the Ospreys' advantage before replacement hooker Richard Hibbard was driven over from a close range line-out. Hook added the extra points.
Gloucester grabbed a consolation score in injury time when Ryan Lamb raced onto a neat kick through by Mike Tindall to make the try.
target=ioMain>Click Here for a Statistical Match Report from the Ospreys v Gloucester Rugby Heineken Cup clash