Nicky Robinson kicked off the second semi final of the day and it was the Blues who put early pressure on the Ospreys with some neat passing in midfield before Hook eventually cleared.
The Ospreys were awarded an early penalty with the Blues going in to a ruck from the side, Hook stepped up but struck the ball just wide of the posts.
The Ospreys did put pressure on the Blues, but a quick pass from Nick Robisnon released Tom James who gained ground going up the wing. The Blues were eventually awarded a penalty but Ben Blair hit the upright but offside from the Ospreys lead to another penalty and this time Blair struck it right between the posts.
Minutes late the Ospreys had worked their way in to the Blues twenty two and a free kick to touch saw the Ospreys win the line out and spread the ball wide. Excellent defence drove Walker in to touch and the Blues cleared.
Scott Morgan was harshly dealt a yellow card for coming in to a ruck from the wrong side and Hook kicked the resulting penalty to level the scores. The Blues were down to 14 men and the Ospreys were applying pressure, however a move broke down in the Blues twenty two and Mike Philips hacked upfield in to the Ospreys twenty two before Hook once again cleared.
The breakthrough however came for the Ospreys when Sonny Parker shock off a tackle and sprinted for the corner to score the first try of the match. Hook added the conversion.
The Blues were dealt a further blow when Deiniol Jones was sent to the sin bin straight from the restart. The Blues were down to 13 men and the gaps were appearing in the defence: it was down to Tom James and Martyn Williams to save the day when the Ospreys looked like scoring.
The Blues were back up to fourteen men with the return of Scott Morgan but this didnt last for long with Rob Sidoli being shown a yellow card. The referee blew for half time and the Blues were seven points behind. It was only courageous defence that stopped it being a bigger margin, the Blues had only played for thirteen minutes with fifteen men.
HALF TIME CARDIFF BLUES 3 OSPREYS 10
Justin Marshall proved to be a handful in the loose and when the Ospreys spread the ball out wide Lee Byrne just managed to squeeze in the corner for the second try of the match. Hook missed the conversion but the lead had extended to 15-3.
With players uncharacteristically dropping off tackles the Ospreys were breaking through the Blues defence. Hook added another penalty to bring the score to 18-3.
The Blues fought back and Mike Phillips quick penalty saw him come close to the Ospreys try line only for them to steal the ball and clear upfield. The Blues drove from the line out and just when they looked to have released a man out wide the ball was passed forward. They were called back for a penalty and elected for the scrum in front of the posts. The Ospreys stole it and cleared their lines it was the story of the afternoon.
A controversial penalty was awarded to the Ospreys when Rhys Thomas was pushed in to one of their players off the ball, Hook stepped up and converted to extend the lead further.
The Blues never gave up and fought back with Jamie Robinson coming in on a great angle and releasing Chris Czekaj down the left. However they failed to capitalise and Hook slotted another penalty.
The score for the Blues came when neat passing in the backs saw Blair released in the corner for a try that he converted. However from the restart Hook struck a drop goal to have the final say on the match and put the Ospreys through to the final where they face Leicester Tigers.