OAKMONT, Pa. (AP) — Dustin Johnson settled the score Sunday in the U.S. Open.

Johnson atoned for his past mishaps in the majors by showing he had the smarts to handle the toughest test in golf, even while playing the final two hours without knowing where he stood when the USGA questioned whether he should be penalized one stroke for his ball moving on the fifth green.

Johnson said it didn’t. The USGA said it would wait until after the round to decide.

America’s most powerful golfer took matters into his own hands at Oakmont, capping off a chaotic and confusing final round by stuffing his approach into 5 feet for a birdie that made the penalty a moot point.

The USGA ended up penalizing him, turning that 68 into a 1-under 69. The score was irrelevant. He won by three shots.

Finally, he’s a major champion.

Johnson scooped up 18-month son Tatum into his arms on Father’s Day and raised the silver trophy for all to see.

“I’ve been here a bunch of times and haven’t quite got it done,” Johnson said. “But today, I did. And it feels really good.”

He saluted a Pittsburgh crowd that was on his side even amid all the uncertainty. The grandstands were raucous, with one fan shouting, “What’s the call, USGA?” At the trophy presentation, when Fox Sports announcer Joe Buck brought up the penalty situation, the crowd booed.

“I just tried to focus on what I was doing, not worrying about the penalty stroke,” Johnson said. “Just playing golf from here to the house.”

He slammed the door at the end — a 10-foot par save on the 16th hole to keep his lead at two shots, a solid par on the 17th as Shane Lowry was self-destructing behind him, and a shot into the 18th that plopped down near the pin and settled 5 feet away for birdie.

“Might be one of the best shots I’ve ever hit under the circumstances,” Johnson said.

He finished at 4-under 276, the lowest winning score in nine U.S. Opens at Oakmont.

The lingering question was whether the toughest test in golf was tougher than it needed to be.

Johnson had a short par putt on the fifth hole, took a few practice strokes and as he placed the putter behind the ball, it moved slightly — backward. Johnson stepped back and called over the rules official, told him he didn’t cause it to move. He tapped in for par.

Jeff Hall, senior director of rules and open championships for the USGA, said a staff member said on the radio that it might be worth another look. The USGA thought Johnson should know that his score might be one shot worse than it was, so it told him on the 12th tee.

“After looking at video, the actions he took could have caused the ball to move,” Hall said. “We asked if there was some other reason the ball could have moved. He didn’t state a reason.”

But it led to confusion over the entire back nine — for Johnson and for the guys trying to catch him.

Lowry, who began the final round with a four-shot lead that he lost on the front nine, tied him when Johnson made bogey on the 14th.

Were they tied? Was Johnson trailing by one?

Jack Nicklaus, who won the first of his 18 majors at Oakmont in 1962, said if the USGA thought it might be a one-shot penalty, it should have done it right there and “let him get on with the job.”

That’s what he did, scrambling for pars, keeping his cool, thinking only the major that kept eluding him. The most painful of all was last year in the U.S. Open at Chambers Bay, when Johnson had a 12-foot eagle putt to win and three-putted for par to lose by one to Jordan Spieth.

There also was that two-shot penalty at Whistling Straits that kept him out of a playoff in the 2010 PGA Championship when Johnson grounded his club in sand without realizing it was a bunker. He was chasing down Darren Clarke in the 2011 British Open when he hit a 2-iron out-of-bounds on the 14th hole. He lost a three-shot lead at Pebble Beach in the 2010 U.S. Open by closing with an 82.

No doubted the talent. Spieth said two weeks ago that Johnson was “arguably the most talented player on the PGA Tour.”

But did he have the patience? The wits? The smarts?

He answered all those questions on an Oakmont course that finally lived up to its reputation as the toughest in the land.

Lowry became the first player since Payne Stewart at Olympic Club in 1998 to lose a four-shot lead in the final round of the U.S. Open. He made birdie after being told of the potential Johnson penalty, but the Irishman lost his putting touch with three-putt bogeys on three straight holes. He closed with a 76.

Sergio Garcia also had another chance until running into trouble with the bunkers and making three straight bogeys. He shot 70 and tied for fifth. Scott Piercy got within one shot of the lead until bogeys on two of the last three holes for a 69 to tie Lowry and Jim Furyk (66) for second.

Furyk made bogey on his final hole just as Johnson was teeing off at No. 10. Furyk was a runner-up at Oakmont for the second straight time. In his final major in his home state, he walked off the 18th green waving his cap toward the crowd with his hand over his heart.

Two hours later, with great shots overcoming peculiar decisions, the stage belonged to Johnson.