The Australian bogeyed the hole while his two rivals birdied, meaning he was virtually out of the running from that point on, but Leishman took the misfortune with good grace.
Zach Johnson is 39-years-old. All three made bogey on 17, again, no surprise, and after Johnson missed his birdie putt on 18, Oosthuizen had an opportunity to tie him and send the playoff to sudden death.
At least he was in elite company. Spieth has won four tournaments this year, including two majors – the Masters and U.S. Open – and he’s finished in the top five in six others.
He is also only the sixth Australian to win the Canadian Open, joining Joe Kirkwood (1933), Jim Ferrier (1950, 1951), Kel Nagle (1964), Greg Norman (1984, 1992) and Nathan Green (2009) on the honour board.
Surely there would be more magic to come and Spieth would enter the record books as the first player to win his first three majors of the year since Ben Hogan did it 62 years ago.
Leishman, who had also covered the front nine in 31 and birdied the 10th 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.
Zach Johnson clinched his second major title after winning the 144 Open Championship in a play-off against Louis Oosthuizen and Marc Leishman at St Andrews.