O'Hern, winless since 1999, had a 16-under 200 total on the Huntingdale course. The left-hander shot a course-record 64 on Thursday and had a 66 on Friday.
'I played very solidly again today,' O'Hern said. 'Just too many putts. It's that simple. I felt I putted all right, I just didn't hit the lines.'
Allenby, the 2004 tournament winner who is trying to become the first player to sweep the Australian Open, PGA and Masters in one season, shot his second straight 68. He birdied the par-4 17th to pull within four strokes of O'Hern and gained another stroke when the leader bogeyed the hole minutes later.
'Obviously I was pleased with the way I played on the front nine,' Allenby said. 'On the back nine I found that I just wasn't hitting it close enough.'
Allenby said the greens on the back nine also caused him problems.
'I struggled purely because of the speed of the greens,' he said. 'They're just inconsistent. I found that the surface ... once we got to nine, the greens just started going a different color.'
O'Hern has 23 top-10 finishes in 51 tournaments in the past two years. He finished third in the HSBC Champions last month in China, was second two weeks ago in the Australian Open and third last week in the Australian PGA.
American Bubba Watson (69) was four strokes back at 12 under, and Australia's John Senden (68) was another shot behind, Australian star Stuart Appleby (66) was 9 under, and England's Paul Casey (68) was 8 under.
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