It was the 39-year-old American’s second major title win after he triumphed at the 2007 Masters.
Johnson fought back tears after the win and was quickly congratulated by Jordan Spieth, eight days after finishing a shot behind his Ryder Cup team-mate in the John Deere Classic.
“It sounds beautiful. It still sounds extremely surreal. The tone to it is very humbling. I feel blessed to be the champion and honoured to be part of the history of this game,” Johnson said.
It all came down to the famous 18th hole with its historic town centre backdrop, and a par was enough for Johnson with Oosthuizen narrowly missing an eight-footer to force sudden death.
“To don my name on that trophy is humbling and surreal,” said Johnson, whose superb wedge-play and putting were key to his win.
“It has been a week of patience, courage and trust. I can’t play any better than I did. I just stayed in it, waited for the opportunities and made a few putts.
“I don’t like seeing it end on a miss. Louis is a buddy, a friend and a tremendous competitor.”
Johnson was also quick to praise Masters and US Open winner Spieth after he agonisingly failed to keep his Grand Slam dream alive.
“I can’t describe the magnitude as to what he (Spieth) was going through because I’ve never been in that position certainly,” added Johnson, who moves from 25th to 12th in the world rankings. “We haven’t really seen that with the exception of Tiger.
“Truthfully he could be sitting here. But to have a champion like Jordan take the time on 18 to give me best wishes speaks volumes as to what he is. He’s a phenomenal talent, and I’m telling you right now, he’s a better person than he is a golfer.”