Richard S Johnson sinks monster putt at Scandanavian Masters
South African Oosthuizen was in contention to become the first player to follow up victory in the British major with a regular tour win but he had to settle for fourth.
Johnson delighted the home crowd at a soaking Bro Hof Slott course near Stockholm as his closing one-under-par 71 for an 11-under 277 total edged out Argentina's Rafa Echenique (69) by a stroke.
The victory, worth $350,000, was Johnson's second on the European Tour, coming more than eight years after he won the ANZ Championship in Australia.
After more than four hours' delay because of heavy rain, Johnson began the final round sharing a one-stroke lead over Oosthuizen with South Korean K J Choi. When Choi immediately hit out of bounds at the first, however, to run up double-bogey, closely followed by two bogeys, the 40-year-old US PGA Tour regular was effectively out of the final mix.
First, Nicolas Colsaerts came through the field to grab the lead. As the Belgian collapsed spectacularly, it was Echenique, looking for his maiden title, who ran Johnson closest.
Echenique, best known for his albatross at the final hole in Munich to finish second in the BMW International and win Europe's 2009 shot of the year, set a 10-under target.
Johnson knew he needed something special and obliged with the lengthy putt on 18 to become the fifth Swede to take the Scandinavian title.
In his rare appearance in Europe, it was a dramatic return to form for Johnson, who had slumped to 161st on the US PGA Tour he has played since 2006 and 329th in the world rankings.
"I actually roared out loud when the putt went in, it meant so much to me," the winner told reporters.
"This is second only to a major for a Swede. It's been a struggle in the States but this is just magical."
Italian Edoardo Molinari (69) finished third a further stroke back to enhance his Ryder Cup chances.
Oosthuizen's form rarely reached the heights of St Andrews last week as he carded a 73 to share fourth with Australian Brett Rumford (69) and New Zealand's Mark Brown (71).
However, the 27-year-old South African was happy he had at least given himself chance to follow up straight away on his maiden major success.
"I definitely wanted to go for it but I didn't drive well enough and it put everything else under pressure. Tiredness affected my focus," Oosthuizen told reporters.
He is now taking a week's well-earned rest before tackling the WGC Bridgestone Invitational and the final major of the year, the US PGA Championship.