The 33-year-old world No. 21 took a third-round three-shot lead into the final day before he finished on 16 under, one shot ahead of Frenchman Alexander Levy — with three birdies and a bogey either end of his round — to claim his eighth European Tour title.
The win by the 2010 Open Championship makes it six victories in 11 events for South Africa this season. Local favourite Jason Scrivener carded 69 to finish third, with Frenchman Gregory Bourdy and Peter Uihlein of the USA a further shot back on 13 under.
Oosthuizen saw his three-shot overnight lead quickly cut to one thanks to a bogey on the second, as Scrivener and Uihlein made early gains, with France’s Romain Wattel holing a brilliant putt for eagle on the third.
South African-born Scrivener had a brief share of the lead after making birdie with a bunker recovery on the seventh but Oosthuizen repeated the trick from the same hazard just minutes later to get back in front.
That birdie seemed to spur Oosthuizen and he put his tee-shot on the par three next to seven feet and rolled in the putt to get to 16 under.
That gave Oosthuizen a three-shot lead when Scrivener left his par putt on the ninth short but Uihlein — after brushing the hole on numerous occasions — got one to drop for a birdie and the gap was two shots at the turn.
Scrivener, Uihlein, and Wattel saw their challenges fade down the back nine and, when Oosthuizen put his tee shot to 10 feet on the 12th, the lead was extended to four shots.
Levy hit a brilliant second into the 18th to set up a birdie and card a 66, leaving Oosthuizen with a two-shot lead to protect up the stretch, which he did with a bogey on the last.
The Frenchman’s round contained eight birdies but also a double-bogey at the sixth.