Oosthuizen toured Sunday's final round at St. Andrews in 1-under-par 71 and finished at 16-under 272, seven strokes clear of runner-up Lee Westwood with another three golfers tied for third at 8-under 280.
Oosthuizen answered the one challenge of the day Sunday -- when Paul Casey got within three strokes at the eighth hole -- with an eagle at the 352-yard, par-4 ninth hole and finished the back nine in even par after Casey fell back with a triple bogey at the 12th hole.
It was only the ninth major tournament appearance for Oosthuizen and only the second time he made the 36-hole cut. The other time he played on the weekend he finished 73rd at the 2008 PGA Championship. He has seven wins as a professional, mostly on the Sunshine Tour, which competes in Oosthuizen's native South Africa.
The first two days of the tournament were dominated by Scotland's winds. Oosthuizen was fortunate enough to play relatively early before the weather was the worst and was at 12-under after 36 holes, which was worth a five-stroke lead.
He met the challenge of playing with a major-tournament lead nicely. After a bogey at the first hole Saturday, he didn't have another until the eighth hole Sunday.