Robert Damron shot a 67 at the TPC at Deere Run to join Hank Kuehne in a tie for second place at 15-under-par 269. Last year's winner Mark Hensby posted a 64 to finish one shot further back alongside Wes Short and third-round leader J.L. Lewis.
The 22-year-old picked up a birdie at the par-5 10th to get within one shot of the lead. O'Hair kept pace over the next three holes and drained an 18-foot putt for a birdie at the par-4 14th to move into first place.
O'Hair maintained his composure over the closing holes and hit his second shot into a front bunker at the par-5 17th. O'Hair played his third shot out of the sand to two feet and tapped in for birdie to reach 16 under.
O'Hair, who was the runner-up at the Byron Nelson Championship, hit his drive in the rough at the par-4 closing hole behind the trees lining the right side of the fairway. He had an opening and knocked his second shot across the fairway and watched as it rolled into the hazard above the water.
He was unable to ground his club for his third, but he played his shot within 7 feet of the hole. O'Hair calmly rolled in the par-saving putt for his maiden title, and a decision on whether or not to go to St. Andrews next week for the British Open.
Kuehne had three birdies over his first seven holes to grab a share of the lead, but he found trouble with a bogey at the par-4 eighth. He countered with a birdie at the par-4 14th, but gave that shot back with a bogey at the following hole.
He reached the green in two at the par-5 17th and two-putted for birdie to get within one of O'Hair. Kuehne hit another huge drive at the last, but sent his second shot over the back of the green. Kuehne was unable to convert for birdie to finish second along with Damron.
Shigeki Maruyama, Richard S. Johnson and Hunter Mahan tied for seventh place at 13-under-par 271. Kevin Stadler, Jeff Brehaut and D.J. Trahan followed at 12-under-par 272.