Fox had a horror start to his round dropping three shots on the first two holes before recording birdies at the seventh and eighth to be one-over at the turn. Two more birdies at the 11th and 14th had him set for another strong round before a second double bogey at the 15th set up a round of 71.
"I just drove too poorly today, to be honest. The rough out there is pretty nasty and I ended up in there a lot," Fox said.
"Got some pretty lucky breaks in there and got a couple really bad lies and missed a couple greens short side, and you can't do that round here...got stung pretty badly when I did that."
"I hit a lot of shots close that I didn't take advantage of. Played the par-fives really nicely. I think I hit two second shots inside 20 feet there and lipped one out and ran one on the edge on the other one.
"Could have been a lot better... just one of those days. It's links golf, and as I said, the margins for error round here are pretty small. The pin placements are tricky today I thought. I was just not quite there.
Fox's previous best finish at a major was tied for 19th at the 2019 Open Championship at Royal Portrush. He will play alongside four-time major winner Brooks Koepka in the final round.
"Yes it probably takes me out of contention for the tournament, but it doesn't take me out of contention for a decent finish. I just need to keep it in play a little better tomorrow and hopefully everything else stays how it is and I've got a good shot."
Oosthuizen has been runner-up in the last two majors and will be trying to win his first major since the 2010 British Open at St. Andrews.
Morikawa was four shots behind when he walked off the 10th green. That's right when Oosthuizen started getting scrappy with his swing. He dropped shots on the 11th and 13th holes. He failed to birdie the par-5 14th. But he made a great par save on the 15th.
Morikawa started poorly with two bogeys in the opening five holes. He was bogey-free the rest of the way and shot 68.
Eight players are separated by five shots. That includes U.S. Open champion Jon Rahm. He made a 15-foot par putt on the last hole for a 68 and was five behind.