The South African started the day at Kiawah island in a share of the lead with Phil Mickelson but surrendered a three putt on the first before picking up shots on the eighth, 11th and 12th.
He had been five shots behind Mickelson stood on the 11th tee but after 13 holes he was just one back, despite dropping a shot on the 13th after finding water off the tee.
He missed a golden opportunity for birdie on the 16th and was punished as he three putted the 17th to sign for a 72 and head into the final round at five under, two shots behind Mickelson.
That could give him an excellent opportunity to add to the Claret Jug he won in 2010, but Oosthuizen feels he will have to be much improved if he is to lift the Wanamaker Trophy.
"I wouldn't call that solid play," he said. "That was probably the worst I've played in a while.
That's why we practise every day and why we play the game, to have a chance to win a Major
"Especially the first ten holes going along with Phil hitting it beautifully and playing great, I was all over the place.
"I could sense early on that I wasn't on song, especially I felt a move in my driver that I didn't like and from there it wasn't good.
"So I was just sort of fighting to stay in it and at the end there, started judging the greens wrong and everything just fell apart.
"All in all, two behind going into Sunday, I've got to take a lot of positives out of that with the way I was playing today.
"That's why we practise every day and why we play the game, to have a chance to win a Major."