Kevin Na (70) and third-round leader Miguel Angel Jimenez (72) shared second place at 2-under-par 286. Trevor Immelman fired a 7-under 65 on Sunday to vault from 14th place to fourth place at 1-under-par 287.
Oberholser tied Jimenez in the lead at minus-2 with a birdie at the par-5 third. The 29-year-old California native quickly fell two shots behind Jimenez as he bogeyed the fourth, while the Spaniard birdied the same hole to get to 3 under.
Oberholser, a two-time winner on the Nationwide Tour, slid three shots back as Jimenez birdied the sixth. Things quickly turned in Oberholser's favor. He birdied the seventh and eighth to get to 3 under.
Jimenez, meanwhile, faltered to bogeys at the eighth and ninth to drop to minus-2 and one behind Oberholser. Jimenez dropped two back as he bogeyed the par-4 12th at Jungmun Golf Club.
Oberholser extended his lead with a birdie at the par-3 13th. Both Oberholser and Jimenez birdied the par-5 15th. Oberholser stumbled to a bogey at the 16th to drop to minus-4. He parred his final two holes for the win.
Jimenez could only manage three pars over his final three holes to close his round at minus-2.
Na began his day at even-par two shots behind Jimenez. Na remained right there as he parred his first eight holes. He stumbled to back-to-back bogeys from the ninth.
The 21-year-old Na got one stroke back when he birdied the par-5 11th. He jumped into contention as he ran off three straight birdies from the par-4 14th to get to minus-2. He parred his final two holes to share second place.
Tim Petrovic moved into a tie for fifth at 1-over-par 289 with a final-round 70. Daniel Chopra joined Petrovic at plus-1 with a 1-over 71 on Sunday.
Duffy Waldorf (68), Matt Kuchar (70), Mathias Gronberg (70) and Brian Bateman (74) finished one stroke further back at plus-2.