Immelman opened by blistering the front nine with a 6-under-par 29, and added four more birdies on the back, but it wasn't enough as countryman Otto shot his second straight 68 to win by two strokes.
It was Otto's second win of the season. The other was his victory at the Nashua Masters in November, and helped vault him into to third on the South African Tour's money race. Immelman, on the strength of his two recent wins and two runner-ups, nearly doubled the amount of his closes pursuer, Mark Foster.
Foster finished the tournament in a tie for 22nd with a 1-over-par 289.
Otto withstood the final day charge of Immelman with a mistake-free round, carding two birdies on both the front and back nines and not a bogey on the day. His scorecard showed one bogey in his last 45 holes.
Going into the final round, Otto seemed to be in complete control of the tournament, especially after opening with birdies on his first two holes. That's when Immelman went on his tear, producing six birdies in his first eight holes to close the gap to just three at the turn.
It was tough out there, especially when Trevor had those five birdies in a row. That was a bit of a shock, Otto said. I made two good birdies on the first two holes and I thought that will get me further ahead but they came back at me.
Otto finished the tournament at 17-under 271 while Immelman took second place on 15-under par 273. Craig Lile was third on 279 following a 2-under-par 70.
It was, however, still not enough to deny Otto a hard-fought two-stroke victory as he closed with a 68 to win on 17-under par 271.
The win garnered Otto an invitation to both the NEC Invitational and American Express Championship later this year.
Full Coverage of the South African Tour Championship