Paul Stewart (Peter’s son) and Jack Faulkner anchored the school innings with a fifth-wicket stand of 53 to take Boys’ High from 24-4 to 87.
Faulker top-scored with 37 and Stewart added a valuable 23.
Pirates captain Brad Reynolds continued his good form with the ball, taking 4-40, while Richie Needham took 3-23.
Baxter Mackay and Sam Patterson opened for Pirates and put on 28 runs before Mackay was caught by Paul Stewart off Faulkner for 18.
Pirates then lost the next five wickets for 82 runs, leaving Peter Stewart and Dan Torrie to become the match-winners.
OBR set Horouta a target of 193 to win, thanks to opener Craig Christophers, who was unbeaten on 56, and Hamish Evans, who scored 49.
Keegan Martin made an early breakthrough when he bowled Josh Adams for 16 with the score at 18, but that was the signal for Christophers and Evans to take the attack to the bowlers. They added 93 runs before Evans, who faced 72 balls and hit four fours, was run out.
Christophers’ 95-ball innings included six fours. But from 111-1, OBR were all out for 192. Brad Collins claimed career-best figures of 4-30 off seven overs.
Horouta were never in the hunt, thanks to the pace and accuracy of Jimmy Holden (4-17) and Arun Kurup (3-12).
Horouta play Campion in the bottom-three semifinal on Saturday to decide who plays Boys’ High in the final.
HSOB 317-7 (S Tallott 99 retired, G Dhinga 49; M Briant 2-42) def Campion College 47 (M Naden 20; D Salmon 4-7, M Francis 3-27).
OBR 192 (C Christophers 59no, H Evans 49; B Collins 4-30, C Campbell 2-27) def Horouta 56 (J Holden 4-17, A Karup 3-12).
Pirates 123-7 (Peter Stewart 21no, D Torrie 18no; T Mitchell 2-28) def GBHS 122 (J Faulkner 37, Paul Stewart 23; B Reynolds 4-40, R Needham 3-23).