It was the third tour victory for Oosthuizen, who has last year's British Open among his championships.
He, Manuel Quiros and Chris Wood finished 72 holes tied at 18-under-par 276 and headed to the playoff at the East London Golf Club's 18th hole, a 411-yard, par 4.
Oosthuizen's drive on the playoff hole headed to the rough but bounced out into the fairway. He capitalized with an approach that stopped 10 feet from the hole.
Wood's second shot ended up 25 feet from the pin but Quiros knocked his approach over the green. He got up and down for par and Wood tied him with a two-putt. But Oosthuizen was able to end the tournament by making his 10-footer.
Oosthuizen earned his way into the playoff with a 3-under 70 that included birdies at Nos. 3 and 6, a bogey at the ninth hole and an eagle at the 545-yard, par-5 15th.
Wood fired a 68 Sunday and had seven birdies, with five coming in the seven-hole span from Nos. 6-12, and two bogeys. Quiros closed with three consecutive birdies en route to a 4-under 69.
Steven O'Hara, who had a bogey-free 7-under 66, tied with Charl Schwartzel and Jaco Van Zyl, who each shot 70 Sunday, for fourth one stroke behind the leaders.
Oosthuizen collected $204,500 for the win. Quiros and Wood each won $119,000. The golfers tied for fourth each earned $54,000.