Oosthuizen had a final round of 4-under-par 68 for a 72-hole total of 12-under 276. Branden Grace, who also shot 68 Sunday, came in second at 11-under.
Joost Luiten, with a 1-under 71 in the final round, and third-round leader Tommy Fleetwood, who was even-par 72 Sunday, tied for third at 10-under.
The win was the 12th professional title for Oosthuizen, but first since he won the Volvo Golf Champions last year.
Fleetwood hurt his chances of keeping the lead he took Saturday with three front-side bogeys and Luiten dug an early hole with bogeys at the first and second holes that gave an edge to Oosthuizen and Grace.
Grace, trying to recover from an opening round 74, had three consecutive birdies over Nos. 6-8 to become the first to reach 10-under for the tournament. But Oosthuizen sank a 12-foot putt for eagle at the par-5 eighth hole and moved back ahead.
He gave a stroke back at the 10th but had birdies at Nos. 13 and 14. A bogey at 16 -- a par-4 he took a 7 on Saturday -- dropped him back but he closed with birdies at 17 and 18 for the title.