Jamieson, who played at Augusta State from 2001-2002 through the 2004-2005 season, began the day a stroke behind the former British Open champion.
But he beat Oosthuizen by six shots to finish at 15-under
to lead Oosthuizen (74), Thongchai Jaidee (73) and Julien Quesne (67).
Padraig Harrington (67) and Francesco Molinari (68) were another two strokes back in the event for European Tour winners.
A month ago, in the Nelson Mandela Championship, Jamieson won his first European Tour title. Now he’s one round away from earning the Durban Double.Jamieson said winning Nelson Mandela in a playoff under the extreme pressure “gave me a lot of confidence that I’ve taken into this week.”
Jamieson had an eagle on the par-5 eighth hole after landing the ball 5 feet from the hole with a 5-iron for 198 yards. He also birdied three holes in a row from the 11th.
“It’s been another great day. While I didn’t play as well as I did the first two days, especially yesterday from tee to green, I managed to score well, which is the main thing,” Jamieson said. “So I’m delighted to be in the position I’m in.
For three days, Jamieson has competed against some of the bigger names in golf.
He played the first round with Harrington and finished a shot ahead of him. On day two, he was six shots better than Thomas Bjorn, and was six strokes better Saturday than crowd favorite Oosthuizen.
“You always have your eye out, comparing yourself to them,” Jamieson said. “But yeah, I managed to beat them through the week, so that was nice.”