The Englishman's 11-birdie blitz put him at 18-under-par 198 through three rounds, level with overnight leader Oosthuizen who battled to a 70 on the par-72 Olazabal Course at southern China's Mission Hills.
On a day of red numbers at Dongguan, even Westwood's brilliant round was eclipsed by America's Brandt Snedeker, who shot a course-record 60 — and only narrowly missed a birdie putt on 18 for a magical score of 59.
Westwood and Oosthuizen will go into the last round three strokes ahead of America's Phil Mickelson, a double WGC-HSBC Champions winner and four-time Major champion, who blazed back into contention with a 66.
Ian Poulter and Bill Haas were four shots off the lead along with Ernie Els, who whittled down fellow South African Oosthuizen's overnight advantage before imploding with a watery double-bogey on the par-five 15th.
Snedeker was five off the pace after his astonishing 60, which was all but matched by Westwood as the former world number one started the day nine shots adrift and finished with the joint lead.
"I just wanted to come out fast and try to get some momentum. That's been missing from my game the last couple of weeks," said Westwood.
"I came out as quick as you like with three birdies, and that got me going and the hole started to look bigger."
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