Oosthuizen, the 2010 Open Champion, was unable to add the scalp of Day to his victories over top-ranked opposition including world number one Jordan Spieth in the quarterfinal.
With the win, Day will displace Spieth as the number one player in the world when the rankings are readjusted on Monday.
Oosthuizen, 32, was unable to replicate the form which had seen him go undefeated in his first six contests in the matchplay event.
“You have to play well against Jason. I couldn’t buy a birdie,” said Oosthuizen afterwards.
Despite the loss, Oosthuizen was pleased with the state of his game.
“The game is really shaping up well now ahead of Augusta and I’m feeling confident about my game. I hit the ball well and hit the shots I wanted, so I’ll take a lot of positives out of this week.”
Oosthuizen began the final in the strongest possible fashion by driving the 400-yard par four first hole to go 1-up out the gate.
Oosthuizen, however, would have no response for Day who won four of the last six holes on the opening nine meaning the Aussie turned in 3-up.
As the pair square the 10th, it meant Oosthuizen had eight holes left to try and mount a comeback.
Day struck another killer blow at the driveable par four 13th to go 4-up with five to play. There both players hit their tee shots just over the green, and while Day was able to nestle his chip up to eight feet from the hole, Oosthuizen fluffed his flop shot leaving around 30 feet for birdie. The South African came close to holing the lengthy birdie attempt, but had to settle for par. Day, as he had done with every putt inside 10ft during the round, poured the putt into the hole to just about seal the match as a contest.
On the 14th, Oosthuizen missed the par four putting surface in two and Day fired in a wedge to a couple feet. Oosthuizen was unable to chip in for birdie and was forced to concede the hole giving Day a resounding victory.
African News Agency
Original source: Oosthuizen stumbles at final hurdle