Oldcorn began the final round four stroke ahead but three bogeys in his first five holes saw him overhauled by Australia's Fowler.
Fowler opened with three pars before his first birdie, on the fourth hole. Both picked up a shot at the sixth, sharing the lead on seven under par before Fowler posted back-to-back birdies as Oldcorn double-bogeyed the eighth. Birdies on the 10th and 11th gave Fowler breathing space, before seven pars brought him home in 66 for a total of 205, 11 under.
Oldcorn, who eventually signed for a one-over 73, has finished runner-up to Fowler three times. "I shot myself in the foot," he said. "I was too tentative early on."