Spain's Carlos Rodiles shot a 6-under 66 to finish third while a group of 12 players were tied for fourth at minus 5.
O'Hern began his day on the back nine of the Masters Course at Laguna National Golf and Country Club and quickly got in the red with back-to-back birdies from the 11th. He dropped his only shot with a bogey at the par-5 15th and then birdied 17 before torching the front.
The 30-year-old birdied the first and added back-to-back birdies from the third. On the par-5 seventh, O'Hern made the first of three consecutive birdies en route to the new course record.
'It was unexpected,' said O'Hern. 'I was on the range and hitting the ball kind of ordinary and all week I haven't been hitting the ball that great. Then on the third or fourth hole something clicked. I've been putting well lately so just holed some nice putts which complemented things.'
Pottier played a bogey-free round to finish one back of O'Hern.
'I putted very well today,' said Pottier. 'The greens are not easy but I had a good feeling and read them well with the right speed. I felt confident.'
Rodiles started on the back nine Thursday with a chip-in birdie on the 10th. He added another birdie on the following hole and made two more to make the turn at minus 4. He then birdied the first and the eighth for a flawless round.
'I played very solidly,' said Rodiles. 'Made a very good start which helps. The 10th, my first, was probably the hole I played worst today and I chipped in for birdie from 20 yards. Then birdied the next hole -- the par-5 -- and I was off.
'I think this is a very good course. It was so different today after the wind on the two practice days. Today was playing shorter. It was playing good for scoring. If the wind picks up scores won't be as low.'
Jean Van de Velde was among the 12 players at 5-under. The Frenchman was joined by Davidoff Tour Order of Merit leader Thongchai Jaidee of Thailand among others.
Two-time British Open winner Nick Faldo carded a 4-under 68 to finish four shots off the lead. He was joined by Thailand's Prayad Marksaeng, Ted Oh of Korea and Denmark's Steen Tinning.
Last year's champion Vijay Singh is not on hand this week to defend his title. He is part of the 64-man field competing at the WGC-Accenture Match Play Championship in California.
Full-field scores from the Singapore Masters