Oosthuizen's total of 16-under 128 tied the tournament record for 36 holes. Adam Scott, who shared the first-round lead with Oosthuizen, was 4-under and is even with Ernie Els for second at 11-under. Els went out in 29 and matched Oosthuizen's 63.
Els opened play with three consecutive birdies. He had three more over Nos. 5-7 and closed the front nine with a birdie at No. 9. He was 2-under on the second nine.
Scott was 5-under through 11 but posted back-to-back bogeys at 12 and 13. He had birdies at 15 and 16 but, for the second day, had a 4 at the par-3 17th.
Oosthuizen bogeyed the par-3 second but was 6-under the rest of the way on the front nine. That included an eagle 3 at the seventh hole. He had four more birdies on the second nine.
The South African star is 11-under on the five par 5s on the Olazabal Course at Mission Hills.