The Wales star scored an early stunning solo try as the Ospreys beat Guinness Premiership leaders Bristol 34-3 at the Liberty Stadium last night.
The victory completes a hat-trick of wins over English opponents in this cross-border competition following victories over Gloucester and Bath and sends them into today's draw for the semi-finals to be held in the Millennium Stadium.
Justin Marshall, Huw Bennett, Nikki Walker and Barry Williams all went over as the Ospreys sailed into the last four. Yet it was Hook's try that lit up an average contest and proved most telling.
Space opened before the 21-year-old following Josh Taumalolo's missed tackle and Hook made the most of it, stepping inside Sam Cox, beating Marko Stanojevic with a dummy pass before outpacing Craig Short to the line.
Until then the Ospreys struggled to shake off Bristol despite Justin Marshall's early try, but Hook's try and conversion broke the Bristol resistance.
Huw Bennett crashed over just five minutes later from a driving line-out and Scotland wing Nikki Walker combined power and pace to barge his way down the touchline to score in the corner before Barry Williams pounced late on.
Ospreys: Tries - J Marshall, J Hook, H Bennett, N Walker, B Williams. Cons - J Hook (3). Pen - J Hook.
Bristol: Pen - J Strange.
Referee: T Hayes (WRU)