The 25-year-old golfer from Lubbock, Texas, finished at 4-under 280 to win by two strokes over six other players.
"When I woke up this morning, I just felt like it was going to be a good day," said the 6-foot-2 O'Hair, who won $954,000.
Stewart Cink had led going into the final round but slipped to a 74 and was passed by O'Hair on the back nine.
O'Hair's other win was at the John Deer Classic in 2005.
Notable deaths of 2014 [PHOTOS]
UPI Sports Results for September 19, 2014