Scott stuttered slightly in his back nine after an outward 32, but he is still well in contention in second after his round.
"I missed a few greens out there and misjudged the chips, tricky chips, but still, you know, I'm pretty happy with 68. It wasn't my best stuff out there, but moving in the right direction," said Scott.
"Five shots is a lot when you've got a guy like Louis out in front. He's a player who can get hot and shoot a low score like he did today. If he does that over the weekend, he's going to make it really difficult for anyone to catch him."
Ireland's Shane Lowry is the highest placed of the European contingent in a tie for fourth at 10 under, six behind Oosthuizen.
Of his second-round 68, Lowry said: "I didn't birdie two easy par fives and made a bad bogey on the sixth, but other than that I played lovely all day and gave myself a lot of chances.
"What's not to enjoy? I'm playing a World Golf Championship and I'm up there in the top five going into the weekend."
