Scotland's Stephen Gallacher trailed Oosthuizen by one shot at 12-under par with seven holes of his third round remaining, while Rafael Cabrera-Bello, Danny Willett and David Lipsky were tied for third on 10-under.
Oosthuizen, who also led by a stroke at the halfway stage of the co-sanctioned European and Asian Tour event, is still recovering from losing out to American Bubba Watson on the second extra hole in Augusta in the US Masters on Sunday.
The 2010 Open champion was at 13-under par after 10 holes of the penultimate round when thunderstorms and rain ended play early for the second consecutive day at the Kuala Lumpur Golf & Country Club.
"You never know with the weather here so you are trying to get ahead of the guys just in case all of a sudden it becomes a 54 hole event," Oosthuizen told reporters.
"I feel good now. I made good putts on nine and ten for birdie and I have another birdie putt to come back to in the morning so it feels really good. I am playing well and just have to keep giving myself chances."
The four-time European Tour winner made a 30-hour journey from Augusta National to take part in the $2.5 million tournament in the Malaysian capital and, with his family around, he was still struggling to get over the jet lag.
"The jet-lag has been pretty rough, especially having the kids with us because they are sleeping all day and then coming alive at night," Oosthuizen, 29, who also had to return to the course early on Saturday to complete his second round, said.
"Thankfully my wife has been able to sit up with them at night in the room next door while I try and sleep but I haven't had more than four hours straight at any one time.
"It's a tough week but it was always going to be the case this week. None of that will matter if I can win tomorrow."
Germany's former world number one Martin Kaymer was six shots behind the leader through 12 holes while 2011 Masters champion Charl Schwartzel of South Africa was also trailing by six after 14 holes in the third round.