The 29-year-old former Open champion, who missed out on a second major at Augusta when he lost a play-off to American Bubba Watson, finished 17 under par at Kuala Lumpur Golf & Country Club - three shots ahead of Scotland’s Stephen Gallacher.
Oosthuizen admitted yesterday that the 30-hour journey from Georgia had made things tough, but he coped admirably with the jet-lag this week and carded four sub-70 rounds.
He completed his third round (69) earlier today after storms prevented him from doing so yesterday before keeping his cool to post a final-round four-under 68.
“I thought I was going to be a lot more tired,” he said on Sky Sports 1 after clinching his fifth European Tour crown.
“I played well. The game was there so it makes it a lot easier. I had a good week.
“This morning probably was a bit up and down - not going too well - but then I settled in a bit for the second 18.”
Oosthuizen held a one-shot lead overnight and began the day by producing a steady level-par end to his third round - bogeying 13 before gaining the shot back on 16.
Gallacher briefly joined the leader with birdies on 15 and 16 but a bogey on 17 ensured it was Oosthuizen who led going into the final round.
Birdies on three and five gave the South African daylight over Gallacher and, after his third bogey of the week at the par-four sixth set him back, he responded superbly. He held his nerve to sink testing putts for birdie on seven and 13 and then made the most of a slice of luck in narrowly avoiding the water on 16 to pick up another shot there and open up a comfortable cushion.
Oosthuizen acknowledged the birdie on 13 had been crucial.
“I knew I just needed to get two ahead and just attack it a little bit more and then I made another good one on 16,” he said.
Reflecting on the improvements in his game since his 2010 Open triumph at St Andrews, Oosthuizen said: “I think (winning the major) helped me a lot to get my game where it is at the moment. I’ve just been working quite a bit with my coach Pete Cowen and my game is getting where I want it to be.”
Asked if 2012 could turn out to be a “monster year” for him, he replied: “It probably would have been if I had the green jacket! But I had a good week last week and I’m looking forward to the next major, the US Open, and to the Players Championship in three weeks.”
The victory lifted Oosthuizen to second place in the European Tour Race to Dubai behind US Open champion Rory McIlroy.
Gallacher remained the nearest challenger throughout and signed off with a bogey-free final round of 70, but could not close the gap.
The 37-year-old, who marked a four rather than birdie three on his third-round card, was nevertheless more than content with his week’s work.
“I’m pleased with myself,” he said on www.europeantour.com.
“Only a couple of bogeys throughout the whole week I and even threw in a shot which wasn’t there, I signed for a wrong score this morning. Apart from that, I kept my head. I’m quite proud of myself for that.
“I thought I signed it properly. I signed for a 69 when I shot a 68 - I only found out when I was at the first tee. I thought I was tied for the lead, and then was one behind but luckily it didn’t affect me that much. It’s one of those things that you learn. It didn’t cost me much.
“It was a phenomenal performance [by Oosthuizen] after last week. I thought he may get tired today and obviously he didn’t. Fair play to him - he played great.”
England’s Danny Willett (71), Spain’s Rafael Cabrera-Bello (71) and American David Lipsky (70) were tied for third on 12 under.
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