Brian Harman, Brooks Koepka and Tommy Fleetwood are part of the four-way tie after each fired a 2-under 70.
But for the first time since the rankings were introduced in 1986, none of the world's top three made the cut as defending champion Dustin Johnson joined Rory McIlroy and Jason Day in making an early exit.
This birdie putt on the Par 4, 504-yard 5th hole will go down as one of the greatest putts in U.S. Open history.
"I'm probably going to be six behind teeing off tomorrow, six or maybe even more", Oosthuizen said.
Rickie Fowler, Jamie Lovemark and J.B. Holmes were all one shot back, followed by a group of five at 5 under, including amateur Cameron Champ. The last time Day missed a cut in a major was the 2012 PGA Championship, just a month after his son was born.
Fowler came back to birdie the par-3 ninth to get back to one shot back. Reavie moved to 4-under 140, three shots back of the lead on a bunched-up leaderboard.
Sure, he birdied four of his last six holes to salvage a 71 and finish at 5-over 149. "I felt like I've been playing pretty well all week, and didn't have quite the numbers to show for it". "I'm looking forward to my shot at it".
And they had even more company.
"Yeah, 12 under, I'd have about a 10-shot lead in most Opens", he said.
After opening with a six-under 66, Casey was one off the overnight lead as he went out among the early starters on Friday.
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Casey laid up in the rough, took two chops to get out of more rough behind the 14th green, and staggered away with a triple-bogey 8 that might have ruined his day at Erin Hills.
He ended up with two birdies and no bogeys over his last five holes.
"It's very hard to get get through a major championship, let alone a US Open without a blemish on your card", the 39-year-old told BBC Sport. I tried to take my medicine and then messed up the next hole. I got out of position, but it's the attitude, it's the grit that matters at the end of the week.
"It was important to do that".
"I'm optimistic with where my game is", he said. It sets up really well for me, especially if I'm driving it like I did today. And if that's not enough of a traffic jam at the top, 18 players are separated by three shots to start the third round.
"I was enjoying it", he told BBC Sport.
"I've got a busy summer, so I'm excited to play a lot of golf".
There were plenty of contenders for performance of the day but none better than Hideki Matsuyama, who rang up seven birdies in a bogey-free 65 to race within two shots of the lead.
Casey sandwiched a pair of bogeys around his triple during a four-hole stretch of the back nine. But Thomas had nine birdies and an eagle to break the record and climb to 11 under for the tournament.
Four others are within three shots as the third round winds down. That explains it. But the rising senior at Texas A&M has a spectacular name, absolutely crushes the ball (he leads the field with a 339-yard driving distance average) and is five under through two rounds. He signed for three birdies and two bogeys to finish three under, alongside 2014 US Open victor Martin Kaymer (69) and Austria's Bernd Wiesberger (72). Jon Rahm. Justin Rose.
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