Oosthuizen scattered seven birdies across his card while Scott finished birdie-birdie-eagle to grab part of the lead.
Watson had an up-and-down round with seven birdies and an eagle but also three bogeys while Mickelson posted four birdies and an eagle. Henson had seven birdies and a bogey and Lowry had eight birdies but also bogeys on his final two holes.
Oosthuizen put up the first 65 Thursday. Starting on the second nine of the Mission Hills Golf Course, he made the turn at 3-under and pushed that to 5-under with birdies at Nos. 1 and 3. He finished with birdies at the seventh and ninth holes.
Scott, also opening on the second nine, was 3-under through six holes but had a bogey at the par-3 17th. He got that stroke back at the third hole before his final three-hole charge.
He pulled close to the lead with birdies at 7 and 8 and had his second shot at the 573-yard, par-5 ninth stop 12 feet from the hole. He made the eagle putt to get to 7-under.