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It secured the spot with a solid 28-10 win against rival Southland Boys' High School at Bishopscourt on Saturday.
The game was evenly poised at halftime. Otago Boys' led 8-6 at the break. But it turned up the defensive pressure in the second spell.
No8 Taylor Dale and lock Caleb Hughes inspired that effort.
They mowed down attackers and clocked up tackles in an impressive display.
Warren Loulanting was a wall in the midfield as well.
"It made them think about running down that channel, that is for sure," coach Regan Turoa said.
"Our defence was outstanding."
That helped to set the platform for a breakout try from Michael Manson. Otago Boys' had been defending for a long period of time before coming up with a crucial turnover.
Centre Jake Te Hiwi broke through and passed to Manson, who used his pace to run the try in from about 50m.
Otago Boys' added another to stretch the margin to 18 points and bank a bonus point.
The win has left it in pole position in the Otago Daily Times Cup.
In the other top four match, King's High School reversed an earlier loss to beat John McGlashan College 17-11 in an important away game.
The score was level 11-11 at halftime. But King's knocked over two penalties to clinch the win and keep its prospects alive.
Fullback Ryan Watterston drilled four penalties in a clutch display, and lock Josh Nevin made a big contribution in the lineout and snaffled some key turnovers.
King's will play Southland Boys' in Clinton this weekend in a virtual semifinal.
The winner will earn the right to join Otago Boys' in the final.
In the middle-four competition, Dunstan High School defeated Otago Boys' High School Second XV 19-17 at the North Ground, while Mount Aspiring College dispatched Waitaki Boys' High School 41-19 in Wanaka.
The bottom four produced two tight games.
Wakatipu High School edged South Otago High School 22-12 in Queenstown, while in Mosgiel the Combined Co-Eds beat St Kevin's College 16-10 after trailing 10-3 at halftime.
Replacement winger Tevita Makasini came on and scored two tries to help seal the win. The Combined Co-Eds arguably produced their best defensive effort of the season, which also helped pave the way.