The world No.21 was seven-under through 15 holes in the European and Asian Tour co-sanctioned tournament and stretched his lead out to five shots.
However back-to-back bogeys brought him back to the field on 15-under 201 ahead of Sunday’s final round.
“I think anyone will expect to win, but there’s a lot of golf to be played,” Oosthuizen said.
“You still need to hit the shots. Around this golf course especially, someone can go out and be three or four?under in the first five holes and it changes everything.
“So I need to play good golf tomorrow, solid golf, give myself a lot of birdie opportunities like I did today, and there’s not much more that you can do then.”
The South African, who won the 2010 British Open at St. Andrews, is chasing his 13th career win.
He was runner-up at the 2012 Augusta Masters losing at the second play-off hole to American Bubba Watson and also lost in a play-off at last year’s British Open.
Oosthuizen’s nearest rivals in Perth are American overnight leader Peter Uihlein and Frenchman Romain Wattel, both on 12-under 204.
Moving day didn't disappoint. #PerthIntl
— The European Tour (@EuropeanTour) February 27, 2016
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Wattel kept in title contention with a round of seven-under 65, while Uihlein dropped back with a one-under 71.
Uihlein had four birdies and one bogey on the front nine, then had a triple-bogey on the shortest hole on the course – the downhill par-three 12th, with its tight green.
He got two of those shots back on the 14th and 15th holes, then dropped another on the last to finish one-under for the day.
Wattell’s round had plenty of merit, given he started the day on five-under and had to deal with the onset of a strengthening sea breeze.
Jason Scrivener was the leading Australian after three rounds and was 11-under 205 after carding a three-under 69.
The other overnight leader Australian Brett Rumford lost ground with a one-over 73 and was in fifth spot on 10-under the card.
