Westwood and Oosthuizen were at 18-under 198. Phil Mickelson was three strokes back after a 66. Ernie Els briefly surged into a share of the lead on the back nine before dumping his ball into a reservoir on the 15th hole and taking a double bogey. He shot a 69 and was tied for fourth with Bill Haas (66) and Ian Poulter (65) at 14 under.
Tom Lehman took the lead in the Charles Schwab Cup Championship in Scottsdale, Ariz., and moved closer to becoming the first player to win the Champions Tour's season points title two straight years.
Lehman birdied the final hole for an 8-under 62 and a one-stroke lead over Fred Couples in the season-ending event. Couples also shot a 62, making two eagles.
Lehman shot the lowest round of his Champions Tour career for the second straight day. At 63-62 and 15 under, he was a stroke off the tour record for consecutive rounds set by Bruce Fleisher at 60-64 and 16 under in the 2002 RJR Championship.
Couples is playing for the first time since a back injury forced him to withdraw during the first round of the Boeing Classic outside Seattle in late August.
Lehman was 17 under after three days in perfect conditions on Desert Mountain's Cochise Course. He entered the week second behind Bernhard Langer in the Charles Schwab Cup competition for a $1 million annuity.
Langer, tied for 10th at 6 under after a 70, was a projected 529 points behind Lehman after starting play Thursday with a 211-point lead.
Jay Haas, five strokes ahead of Lehman at the start of the round after matching the tour-record Friday with a 60, had a 69 to drop two strokes back.
South Korea's Lee Bo-mee shot an 8-under 64 to take a four-stroke lead after the second round of the Mizuno Classic in Shima, Japan.
Lee, a regular on the Korean tour, had a 10-under 134 total at Kintetsu Kashikojima. Japan's Rikako Morita was second after a 68.
American Angela Stanford shot a 71 and was tied for third at 4 under with South Korea's Na Yeon Choi, France's Karine Icher and Japan's Ayako Uehara.
Top-ranked Yani Tseng was 3 under after a 70 in the event sanctioned by the LPGA Tour and Japan LPGA.