It secured a four-try bonus point and kept King's scoreless during an impressive second spell.
But the opening half was a nervous display.
South Otago found itself trailing 5-0 at the break after what coach Alan Hill described as a "jittery'' performance.
"They were a bit nervy in the first half but made some good decisions in the second half,'' he said.
South Otago has made a few positional switches since last season but Hill said he might have played up changes in the media earlier in the week, when in reality he was pretty happy with how the side had been going during the pre-season.
The team certainly produced a more polished performance once the nerves were out of the way.
South Otago dominated the scrums, with hooker Brady Robertson securing "four or five'' tightheads.
Midfield back Chase Owen had a fine game and nabbed a nice try.
Winger Delaney McKenzie was lucky enough to finish the game's best try.
Openside flanker Sam Fischli grabbed a steal at the back of the lineout on South Otago's 10m line.
He passed to No8 Josh Hill, who hit the ball up and set up a ruck.
It was spun to Luc Thomson, who chipped ahead, and McKenzie got the bounce, bumped a defender and ran in to score.
The game was tightly contested and the final score was a little flattering for South Otago, Hill said.
But he was very pleased with Robertson, who had a brilliant game.
Josh Hill had a strong game in the lineout and lock Zach McKenzie had a busy game as well.
For King's, midfield back James Te Pairi was elusive and created some headaches for South Otago's defence.
In the other matches, Waitaki Boys' High School thumped St Kevin's College 62-0, Kavanagh College edged Taieri College 15-14 and Dunstan High School was too good for Mount Aspiring College, winning 46-12.
