Oosthuizen, the second-round leader and 2010 British Open champion, birdied three of his last four to shoot a six-under 67, while van der Walt, who birdied his first four holes, fired an eight-under 65 Saturday.
The pair finished at 21-under 198 and matched the 2009 54-hole tournament record of James Kingston and Darren Clarke. Oosthuizen and van der Walt are one shot clear of two-time U.S. Open champion Retief Goosen, who carded a seven-under 66 at East London Golf Club.
Danny Willett managed a 65 on Saturday and is fourth at minus-19, followed by Richard Sterne, whose nine-under 64 was not only the low round of day, but it vaulted him to fifth place at 17-under 202.
MORE TO FOLLOW.