It will host St Peter's College (Gore) in the Highlanders area final on Saturday. The winner will play off against the champion team from the Crusaders region for a spot in the national top-four finals.
South Otago led 17-13 at halftime in a tight fixture. The visiting side had the better of the breakdown, while the home side's backs looked the more threatening of the two.
South Otago scored what proved to be a crucial try about two minutes before the break.