Southland Boys' did not lose a game in the competition and coach Alan Bailey said his side played its best match of the season on Saturday.
First five-eighth Scott Eade had a fine game for the home team, scoring 20 points and controlling play.
Others to stand out for Southland Boys' were blindside flanker Stephen Clegg and centre Kieran Short.
The home side led 17-7 at the break and pulled away in the second half, scoring seven tries in all.
Southland Boys', which won the title last year, will play St Bede's College in Christchurch this Saturday to decide the South Island representative for the top four competition.
Otago Boys' suffered from a lack of ball on Saturday and tired near the end of the game as the weight of tackling proved too much.
Backs Lance Taylor and Marcus Tanuvasa scored the tries for Otago Boys', while loose forward Brodie Hume had a fine game for the visitors.
In the battle for the middle four, a try from hooker Chris Schaaf with about 15 minutes to go clinched the win for John McGlashan College over Dunstan High School in Dunedin.
Schaaf charged over after a few attempts at the line by McGlashan forwards to give his side a 13-8 win.
Earlier, Dunstan scored a try from a charged-down kick.
After a hotly contested first half, McGlashan led 3-0 on its home park.
Hamish Sutherland had a strong defensive game at fullback for John McGlashan.
In the bottom four, St Kevin's College beat South Otago 13-0 in Oamaru.
St Kevin's was up 10-0 at the break after lock Peter O'Malley scored the only try of the game.
The match was a torrid affair featuring solid defence from both sides.