Oosthuizen led by five strokes after two rounds but that lead wilted under Westwood's blistering 11-birdie round. Westwood had four consecutive birdies over Nos. 14-17 to finally track down Oosthuizen, who shot 2-under 70 Saturday. They are at 18-under 198 after 54 holes.
Phil Mickelson had four birdies -- and a bogey -- to open the back nine en route to a 6-under 66 and is in third at 15-under. Ernie Els (69 Saturday), Bill Haas (66) and Ian Poulter (65) are tied for fourth at 14-under.
The round of the day was turned in by Brandt Snedeker who jumped from 38th to seventh with a 10-birdie, one-eagle round Saturday for a course record 60. His putt at 18 for a 59 missed to the right and he finished with a par. Snedeker is at 13-under 203 for the tournament.
Oosthuizen had a pair of birdies and a pair of bogeys on each side Saturday, with three of the birdies on par 5s. He's 14-under on the par 5s this week.
Westwood had birdies on all five par 5s Saturday. He opened the round with three straight birdies and added birdies at the other odd-numbered holes for a front nine of 6-under 30. He birdied No. 11 -- a par 5 -- before his run of four birdies over 14-17.
Reindeer recovered after escaping from Santa during lighting ceremony
'SNL': 'Anchorman 2' cast, One Direction sing 'Afternoon Delight' [VIDEO]