The Masters runner-up birdied the 10th hole to move to 13 under at the Kuala Lumpur Golf and Country Club before play was stopped for the day yesterday.
Stephen Gallacher of Scotland was a shot behind after completing 11 holes, while Pablo Larrazabal of Spain was the clubhouse leader at 7-under 209 after shooting a 67.
Most of the top 30 players were still on the course when play was halted.
Rain and lightning also forced play to be called off early during Friday's second round, and Oosthuizen said he was playing aggressively to stay atop the leaderboard in case the final round has to be called off.
“I am probably going for a few more shots than usual in the third round because I am kind of playing it like it might be the last round,” the South African said.
“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. I actually don't think that will be the case now, but there was a point on the sixth hole when I was still thinking there was a chance that this could be the last round.”
Oosthuizen, who lost a playoff to Bubba Watson at Augusta after making a spectacular double eagle in his final round, found sand off the tee at the first three holes but managed to save par on every occasion. He mixed four birdies with a bogey on the sixth to sit at 3 under for his round.
“I feel good now,” he said. “I made good putts on nine and 10 for birdie and I have another birdie putt to come back to at the 11th in the morning so it feels really good. I am playing well and just have to keep giving myself chances.” Oosthuizen will be chased on the final day by the two men who dominated the fourth round of the Dubai Desert Classic in February.
Spain's Rafael Cabrera-Bello, who won in Dubai, was three shots off the lead with five holes of his second round to complete.
Gallacher, who finished second in the desert, has some problems to overcome when he returns to the course. He hooked his drive at the 12th hole wildly into the trees and did not have the chance to play a recovery shot before play was called off.
In Hilton Head Island, South Carolina, Colt Knost shot a 6-under 66 on Friday to take a two-stroke lead over Carl Pettersson after the second round of the RBC Heritage.
Knost, tied for the first-round lead with Chad Campbell and Vaughn Taylor after a 67, had three birdies in a four-hole stretch midway through the round and finished at 9-under 133.
Pettersson had a 65, his best ever showing in 32 career rounds at Harbor Town Golf Links. Two-time RBC Heritage winner Boo Weekley (66) and Harris English (68) were another shot off the lead at 6 under.
World No. 1 Luke Donald rebounded from an opening 75 to shoot 69 and slip inside the 2-over par cut line. Donald needs to finish eighth or better to stay ahead of No. 2 Rory McIlroy.
