The defending champion moved into the lead when his tee shot on the par-3 eighth hole stopped an inch from the hole for an easy birdie. The near-ace put him at 5 under, one shot ahead of Oosthuizen.
Then his smash-and-hack approach — one that worked when he won at Winged Foot last year — came back to haunt him.
DeChambeau had consecutive bogeys at Nos. 11 and 12, and made a mess of the par-5 13th, taking a triple bogey after his feet slipped twice.
It got even worse on the par-4 17th, where he hit into the canyon left for a penalty stroke and shanked an awkward-stance chip shot on his way to a quadruple bogey. He shot 6-over 77 and finished at 3 over, nine shots behind Rahm.
“I’ve had plenty of times where I hit it way worse than today and I won,” DeChambeau said. “It’s just one of those things where I didn’t have the right breaks happen at the right time.”
Rory McIlroy was in position to win his second U.S. Open before being derailed by a double bogey on the par-4 12th hole. He shot 73 to tie for seventh at 1 under, leaving him still without a major championship win since the 2014 PGA Championship.
Matthew Wolff started the day three shots off the lead at 2 under, but never really found a groove in the final round, shooting 74.
Russell Henley shot 76 and fellow overnight co-leader Mackenzie Hughes had a 77.