Oosthuizen was in the lead alone for most of Sunday’s final round after making a double-eagle 2 on the par-5 second hole.
Watson caught up with him by putting his tee shot to eight feet on the 16th hole and making the putt for his fourth straight birdie.
They were playing together in the second-to-last pairing, just ahead of Phil Mickelson, who was two shots back at 8 under, tied with Matt Kuchar.
Lee Westwood had the lowest score in the clubhouse _ 8-under 280 after a round of 68.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP’s earlier story is below.