The South African, who flew in from his home in Florida on a private jet provided by the organisers of the European Tour event, was four-under through 16 holes in Seoul.
After a day when play was suspended for more than two hours due to rain and poor visibility, Oosthuizen trailed clubhouse leaders Jean-Baptiste Gonnet of France, Australian Kieran Pratt, Swede Johan Edfors, Briton Matthew Baldwin and South Korean Kim Gi-whan who all shot 67s.
Oosthuizen roared to the turn in 30 strokes thanks to six birdies in the opening eight holes including four in a row from the first.
The 2010 Open champion, who missed the cut at the US Masters earlier this month, then dropped back with bogeys at the 10th and 12th.
"It's a good start," Oosthuizen said at the Blackstone Golf and Country Club. "I'm hitting it really well and hit it fairly close on the front nine.
"When I teed off on 10 the wind and the weather came in ... but I'm feeling good. I haven't hit it as well as I did on that front nine for a while."
Ireland's Peter Lawrie and South Korea's Lee Jung-hwan carded 68s while Briton Tommy Fleetwood was also four-under after 17 holes.
Former US PGA Championship winner Yang Yong-eun of South Korea posted a 70 in the sixth edition of the event that is also co-sanctioned by the Asian Tour.
"I'm quite used to the hot weather of Thailand, Malaysia and Singapore and also the chilly weather of Korea and the United States," Yang said.
"I know I need to overcome those difficulties...to perform well. However, it was not that easy out there today."