Oosthuizen had an 8-under 64 Friday on a rain-softened Durban Country Club course for a 12-under 132 total, one shot ahead of Scotland's Scott Jamieson (64) and Thailand's Thongchai Jaidee (68).
By sinking the last putt, Oosthuizen's team won a pro-am event that awarded a car to the winners. However, the farming enthusiast talked to organizers and swapped the car for an excavator, which happened to be the prize for anyone who made a hole-in-one at the 15th.
Six others trail Thongchai and Jamieson by six shots.
Oosthuizen added to his collection of agricultural equipment - after raising a few eyebrows when he won the 2010 British Open and spent a large chunk of his prize money on a customized tractor for his farm. On Friday, the South African, Thongchai and an amateur player won the pro-am event.
"I've been nagging my wife for a few years that I want something on the farm as there are a few stumps and things I need to get rid of," Oosthuizen said. "And that's why I stood over that putt on 18 for a bit longer than normal. So I'm going to play around with it and might dig out a few bunkers."
Oosthuizen owns a farm near Mosel Bay on the southern coast of South Africa.
Meanwhile, Jamieson said he's focused on winning in Durban for a second time in a month. He recently won Nelson Mandela Championship at the nearby Royal Durban course
"I'm definitely comfortable playing here in Durban and the confidence is high as well, which is all-important," he said.