But British Open debutantes Jin Jeong of South Korea and Alejandro Canizares of Spain each served notice they are contenders with birdies at the 18th hole on Saturday to seize a share of third on six-under 138, six behind Oosthuizen.
Thirty players in the field of 156 were forced to finish their second rounds in light wind Saturday. Brisk breezes Friday halted play for 65 minutes and forced an early wake-up call for late starters.
Oosthuizen, who missed the cut in seven of his previous eight majors, took advantage of benign early conditions in the second group Friday morning to fire a five-under par 67 and stand on 12-under 132 after 36 holes.
US veteran Mark Calcavecchia, the 1989 Open champion, was second on 137 after a second-round 67. He was the first man to tee off in round two, staying ahead of blustery winds that blew other contenders down the leaderboard.
World number one Tiger Woods and first-round leader Rory McIlroy were among those who struggled in Friday gales. Woods fought to a one-over 73 and is eight strokes back. McIlroy fired an 80 after an opening 63 to fall 11 off the pace.
Jeong, a 20-year-old who won the British Amateur crown last month, and Canizares, son of four-time Ryder Cup starter Jose Canizares, joined Englishmen Lee Westwood and Paul Casey on 138. Casey and Westwood both finished Friday.
Jeong played 17 holes Friday then birdied 18 to finish a round of two-under 70, becoming the only amateur to make the cut.
Canizares was five-under with six holes to finish on Saturday and made five pars before his birdie concluded a round of 71.
Northern Ireland's Darren Clarke, the 1997 Open runner-up who played his way into the field last week at the Scottish Open, also birdied the 18th Saturday for a 70 to join a pack in 14th on 140.
England's 657th-ranked Steven Tiley, the low man on the course at six-under with eight holes remaining when play resumed at 6:30 a.m., began with a double bogey and followed with five bogeys to finish 36 holes on one-over 145.
The third round was set to start later Saturday morning with 77 inside the cut at two-over 146. Those missing the weekend include five-time Open champion Tom Watson and three-time major winners Ernie Els and Padraig Harrington.