Oosthuizen, who finished runner-up in the Masters to American Bubba Watson in a play-off, registered a total of 17-under 271, three ahead of Scotland's Stephen Gallacher.
The 2010 Open champion, who made a 30-hour journey from Augusta National to take part in the $2.5 million event in the Malaysian capital, fired a four-under 68 in the final round to win his fifth European Tour title.
"It would have been better if I had the green jacket," said Oosthuizen, whose daughter rushed out to congratulate him after the win, with a laugh.
"I thought I was going to be a lot more tired. It was a long journey to get here and I have to be honest and say that I didn't expect to play this well because of the tiredness."
Having the family around in the Malaysian capital meant he got little sleep and struggled to get over the jetlag, but on Sunday he was not complaining.
"The one thing I did know was that the game was there so that helped a lot," he said.
"I was pretty tired at points in those first couple of rounds but overall it has been great and I am over the moon with the result.
"It was just a question of whether or not I would be able to go all the way physically.
"I didn't want to come here and play bad but to win means a lot because I have been playing well for the last few weeks now and to win gives me a lot of confidence for the rest of the season."
The 29-year old Oosthuizen had to return to the Kuala Lumpur Golf & Country Club early on Sunday to complete the remaining eight holes of his third round after thunderstorms and rain had ended play early for the second consecutive day on Saturday.
Gallacher, 37, who signed a wrong score in the morning during his third round, hit a bogey-free round of two-under 70.
"I'm pleased with myself, only a couple of bogeys throughout the whole week and even threw in a shot which wasn't there," Gallacher said.
"Signed for a wrong score this morning. Apart from that, I kept my head. I'm quite proud of myself for that."
Rafael Cabrera-Bello of Spain, Danny Willet of England and Asian Tour rookie David Lipsky of the US shared the third spot, five shots behind Oosthuizen.
2011 Masters champion Charl Schwartzel of South Africa finished sixth while Germany's former world number one Martin Kaymer finished tied seventh at nine-under.