O'Hern has been a regular on the tour since 2005 and he is still looking for his first victory. He made eight birdies in a stretch of 11 holes and did not suffer a bogey at the CordeValle Golf Club.
His best finish this year has been a tie for third at the FedEx St. Jude Classic, but he has missed the cut 12 times in his 23 appearances.
Nicolas Colsaerts, a member of the recently victorious European Ryder Cup team, shared second place after one round with Jhonattan Vegas at 65 while those at 66 included Charles Howell III, John Mallinger, Jonas Blixt, Gary Woodland and Greg Owen.
Woodland currently ranks 123rd on the money list and needs a good finish late in the season to secure his spot among the 125 players who will be exempt for next year's tournaments.