For O'Hair, who wound up at 16-under 268, it was his first career victory on the PGA Tour.
Robert Damron shot a 67 to join Hank Kuehne in a tie for second at 269. Last year's winner Mark Hensby had a 64 Sunday to finish one shot further back along with Wes Short and third-round leader J.L. Lewis.
O'Hair, 22, was five shots down at the start of Sunday's play.
Second at the Byron Nelson Championship, O'Hair had to save par out of a hazard on the final hole to capture his first PGA title and a bid to play in next week's British Open.