O’Hern had eight birdies in an 11-hole stretch and capped the bogey-free round with a birdie on the par-5 ninth hole at CordeValle Golf Club.
Colsaerts is trying to earn a PGA Tour card. As a special temporary member, he needs to finish the equivalent of 125th on the money list to earn a full 2013 card. The Belgian player has earned $652,886, enough now for the 120th spot.
Augusta native Charles Howell opened with 66 and is tied for fifth place.
Howell birdied five holes in a six-hole stretch on the front nine to move to 5-under. He birdied No. 16 before making his lone bogey of the day one hole later.
WORLD GOLF FINALS: In Belek, Turkey, Justin Rose beat Tiger Woods by a stroke to set up a title match against Lee Westwood.
The two Englishmen, teammates on the victorious European Ryder Cup team, will meet Saturday, with the winner getting $1.5 million and the runner-up $1 million in the eight-player exhibition.
Rose shot 2-under-par 69 to beat Woods.
Westwood had a course-record 61 to beat 2011 Masters Tournament champion Charl Schwartzel by six shots at Antalya Club.
Earlier Thursday, Woods shot 64 and beat Rory McIlroy by six strokes to reach the semifinals.
EUROPEAN TOUR: In Vilamoura, Portugal, Ross Fisher and Stephen Gallacher each shot 6-under-par 65 to share the lead at the Portugal Masters after the opening round.
Former Augusta State golfers Jamie Elson and Scott Jamieson both had 71s.
Martin Kaymer, who holed the putt that kept the Ryder Cup with Europe less than two weeks ago, shot 69 on the Oceanico Victoria Golf Course.
Ryder Cup teammate Francisco Molinari finished at par. Europe captain Jose Maria Olazabal was 4-over.
LPGA TOUR: In Kuala Lumpur, Malaysia, defending champion Na Yeon Choi shot 6-under-par 65 in the LPGA Malaysia for a share of the lead with Hall of Famer Karrie Webb.
Japan’s Mika Miyazato, South Korea’s Sun Young Yoo and 17-year-old Taiwanese amateur Min Lee are a stroke back at 66.
WEB.COM TOUR: In Miami, former Pepperdine golfer Jason Allred shot 7-under-par 64 to take a one-shot lead after the first round of the Miccosukee Championship.
Augusta’s Scott Parel and Aiken native Matt Hendrix opened with 70s.