"I feel blessed to be the champion," said the 39 year old, who fought back tears after the win and was quickly congratulated by Spieth, eight days after finishing a shot behind his Ryder Cup team-mate in the John Deere Classic.
"I feel honoured to be part of the history of this game and to don my name on that trophy, especially with the names before me. Humbling and surreal are two words that come to mind. I am fairly speechless right now.
Dreams have been realised and goals accomplished. - Johnson
"I can't play any better than what I did. I had a lull on Friday and Saturday but nothing significant, but stayed in it and waited for the opportunities and made a few."
Johnson began the final round three shots behind leaders Oosthuizen, Jason Day and Irish amateur Paul Dunne, but birdies on the second, fourth, fifth, seventh and ninth took him to the turn in 31.
Australia's Adam Scott matched that front nine and the pair also birdied the tenth, but as Scott's challenge collapsed on the back nine, his compatriot Leishman moved to the front.
Leishman, who had also covered the front nine in 31 and birdied the tenth and 12th, briefly held a two-shot lead when Johnson bogeyed the 17th, his right foot slipping on his second shot as a rain shower passed through.
But as Johnson rolled in a long birdie putt on the 18th, Leishman was failing to get up and down from a bunker on the 16th and the 31 year old from Warrnambool could not find a decisive birdie of his own on the last.
Spieth's chances of a third consecutive Major victory looked to have disappeared after he four-putted the eighth - sending his first putt off the green - to fall three behind, but the 21 year old birdied the ninth and tenth and holed from 50 feet on the 16th for another to tie the lead.
Johnson, preparing for a possible play-off on the practice ground, heard the roar and checked his phone to confirm what had happened but it was then Spieth's turn to bogey the 17th after mishitting his approach and coming up well short of the green.
The Masters and US Open winner pulled his drive on the 18th near the grandstands on the right-hand side of the first fairway and had to back off his second shot due to noise from spectators taking pictures.
And after regaining his composure the World Number Two - who would have overtaken the absent Rory McIlroy in the rankings with a victory - saw his approach spin back off the green into the Valley of Sin, from where Costantino Rocca famously holed 20 years ago to get into a play-off with John Daly.
However, this time there were no heroics and the birdie attempt slipped just past the hole, while playing partner Day left his birdie attempt to also get into the play-off inches short.
"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 Official World Golf Ranking. "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."
"I feel blessed to be the champion," said the 39 year old, who fought back tears after the win and was quickly congratulated by Spieth, eight days after finishing a shot behind his Ryder Cup team-mate in the John Deere Classic.
"I feel honoured to be part of the history of this game and to don my name on that trophy, especially with the names before me. Humbling and surreal are two words that come to mind. I am fairly speechless right now.
Dreams have been realised and goals accomplished. - Johnson
"I can't play any better than what I did. I had a lull on Friday and Saturday but nothing significant, but stayed in it and waited for the opportunities and made a few."
Johnson began the final round three shots behind leaders Oosthuizen, Jason Day and Irish amateur Paul Dunne, but birdies on the second, fourth, fifth, seventh and ninth took him to the turn in 31.
Australia's Adam Scott matched that front nine and the pair also birdied the tenth, but as Scott's challenge collapsed on the back nine, his compatriot Leishman moved to the front.
Leishman, who had also covered the front nine in 31 and birdied the tenth and 12th, briefly held a two-shot lead when Johnson bogeyed the 17th, his right foot slipping on his second shot as a rain shower passed through.
But as Johnson rolled in a long birdie putt on the 18th, Leishman was failing to get up and down from a bunker on the 16th and the 31 year old from Warrnambool could not find a decisive birdie of his own on the last.
Spieth's chances of a third consecutive Major victory looked to have disappeared after he four-putted the eighth - sending his first putt off the green - to fall three behind, but the 21 year old birdied the ninth and tenth and holed from 50 feet on the 16th for another to tie the lead.
Johnson, preparing for a possible play-off on the practice ground, heard the roar and checked his phone to confirm what had happened but it was then Spieth's turn to bogey the 17th after mishitting his approach and coming up well short of the green.
The Masters and US Open winner pulled his drive on the 18th near the grandstands on the right-hand side of the first fairway and had to back off his second shot due to noise from spectators taking pictures.
And after regaining his composure the World Number Two - who would have overtaken the absent Rory McIlroy in the rankings with a victory - saw his approach spin back off the green into the Valley of Sin, from where Costantino Rocca famously holed 20 years ago to get into a play-off with John Daly.
However, this time there were no heroics and the birdie attempt slipped just past the hole, while playing partner Day left his birdie attempt to also get into the play-off inches short.
"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 Official World Golf Ranking. "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."
