The home school scored four tries to beat rival King's High School 22-6 in the Highlanders First XV competition.
The unbeaten Southland Boys' High School team leads the top section of the competition after beating Gore High School 7-6 in Gore.
The key game this Saturday is between Otago Boys' and Gore, with the winning team meeting Southland Boys' in the final in Invercargill on August 21.
Centre Makasua Tanavasa scored two of the four Otago Boys' tries with strong running and his ability to break the line.
He scored the only try in the first spell when fullback Liam Edwards hoisted an up-and-under, ran up and caught the ball, and unloaded to Tanavasa to give Otago Boys' a 5-3 lead at the break.
The King's forwards had the edge in the first half and camped inside the Otago Boys' 22m for much of the spell.
Tanavasa gave Otago Boys' the momentum when he breached the King's defensive line midway through the second spell and ran 30m to score.
Halfback Kurt Hammer used his strength to force himself over the line after a forward drive close to the line, and Edwards used his kicking skills to create the bonus-point try late in the game with a superb cross-kick to the waiting hands of No 8 Brodie Hume.
Otago Boys' had field position throughout the second spell, kept the pressure on and forced Kings into errors and turnovers.
The other key player for Otago Boys was second five-eighth Michael Collins, who was a rock in midfield with his organisation and strong defence.
Players to impress for King's were No 8 Junior Fa'avae, prop Ayden Lloyd and halfback Caleb Gray.
Conditions were atrocious for the Southland Boys' game against Gore and the players became mud-spattered and the numbers unrecognisable after just 10 minutes.
The mud and rain led to numerous mistakes by both teams, and it was a game played in the forwards.
Keanu Kahukura scored the only try of the game and the conversion by Scott Eade gave Southland Boys' the narrow win.
First-half tries to Braydon Pennycook and Mark McKenzie gave John McGlashan a 14-0 win over Waitaki Boys' in Dunedin.
First five-eighth Alan Steel added the conversions.
McGlashan is unbeaten in section two of the competition and is set to host the final in Dunedin.
In section three, South Otago came back strongly in the second spell to beat Kavanagh College 14-7 in Balclutha with John Faamoe and Karl Still scoring the tries.
