The home side led 15-10 at the break but St Bede's had clawed its way back from 15-0 and was winning the possession and territory battle.
Then a piece of individual brilliance from hooker Ricky Jackson turned the game. Otago Boys' swung the ball left to Jackson, who showed great pace and swerved and fended his way past two defenders on a 50m dash before making a wonderful overhead pass to Isimeli Tuivaga.
The busy loosie dotted down and the conversion made it 22-10.
Moments later, Jackson made another impressive carry and a well timed pass in a move which finished with 135kg prop Sione Asi crashing over.
The next try was arguably the best. In a planned move, Asi appeared to be setting up a ruck in midfield but offloaded to fullback Josh Buchan.
He ran straight through a gap, stepped one opponent, sold a dummy to another and finished off with an angled run to the corner.
Asi got a second try before fulltime. His side rumbled a maul 20m before it was stopped. But Asi used his bulk to get the ball across the line. Otago Boys' coach Ryan Martin was a little surprised by the winning margin.
''I think our preparation for this game has been as good as any in the last five years,'' he said.
''We have some very good players in the right positions and we came in very confident.
''It [the score] probably flattered us a little bit . . . but in saying that, I thought we were reasonably dominant across all aspects.''
St Bede's scored two first half tries, the second on the stroke of halftime following a prolonged period of attack.
Otago Boys' was under some pressure at the start of the second spell but its response could not have been better. And the defensive effort, particularly in the second half, was mighty.
''We really back our strength and conditioning and we know we can always go to another level,'' Martin said.
''We play 80 minutes in the premier colts [grade] and these interschools are 70 minutes, so we always have 10 minutes up our sleeve.
''And that is what happened and that's what the message was at halftime. Just lift the tempo a wee bit and these guys will be gone.''
The wins means Otago Boys' has qualified for the top four and will head to Rotorua to play Scots College on Friday. Scots College beat St Patrick's College 23-19 in the final of the Hurricanes competition.
St Kentigern College plays Rotorua Boys' High School in the other semifinal, also in Rotorua.
''We'll go up there pretty confident in knowing what we can do to be honest,'' Martin said.
In the South Island co-ed final, South Otago High School gave a spirited performance but was beaten 31-19 by St Andrew's College in Christchurch.
First XV
The scores
Otago Boys' High School 42
Sione Asi 2, George Thomas, Kabe Calder, Isimeli Tuivaga, Josh Buchan tries; Buchan 3 con, 2 pen
St Bede's College 10
Tyler McKendry, Charles Alaimalo tries
Halftime: Otago Boys' 15-10.
