Dustin Johnson walks onto the 18th green at Oakmont. (David Cannon/Getty Images)

Dustin Johnson faced a slippery, sliding, left-to-right, four-foot par putt on the 71st hole of the U.S. Open at Oakmont Country Club on Sunday while clinging to a two-shot — or was it a three-shot — lead.

Normally, or at least for the previous 115 U.S. Opens, the man on the verge of winning his country’s national title, or blowing it for the second straight year, would actually know his own score. But not D.J. Not with the USGA — the United States Golf Association, or is it the United States Goofball Association? — on the loose.

Actually, no one knew where they stood in real time, neither Johnson — who won with a 4-under-par 276 — nor his competitors, because a possible penalty stroke hung over Johnson’s head for the final two hours of play until USGA officials finally made a decision after his round. As outraged or disbelieving fellow pros, including Rory McIlroy and Jordan Spieth, filled the social media world with acerbic tweets like “amateur hour for the USGA” and “this is a joke,” Johnson perfectly maintained the poise and focus that some doubted he possessed.

A year ago, on the 72nd hole of the same tournament, Johnson faced a similar four-foot putt, another left-to-right slider, to get into a playoff with Spieth. That day, Johnson missed, three-putting from 12 feet to go from a possible victory to sudden and completely embarrassed defeat.

This time, Johnson plopped his slider — exactly the type that has always been his nemesis — into the cup, giving himself breathing room. Go on, USGA, do your worst or wackiest.

Johnson’s towering, titanic drive down the center of the 484-yard par-4 18th hole caused slight alterations in the orbit of the earth. His 190-yard 6-iron shot finished barely a pace from the final hole — stone dead, like criticism of the 31-year-old, ranked sixth in the world entering this week, who has often been dubbed the Best Golfer Not to Win a Major Championship.

That last birdie made the USGA’s final ruling — they nailed Johnson for a stroke penalty because his ball rotated perhaps a 16th of an inch on the fifth green as he prepared to putt — totally moot. As a result, Johnson’s score will go down as a grand 69 (not 68) for a 4-under-par total on a course that many of the world’s top players predicted would not allow a sub-par winner.

“I couldn’t be happier or prouder of myself, especially with the things that happened to me on the 18th hole last year when I couldn’t get it done,” said the normally phlegmatic Johnson, who broke into a grin to add, “It was a lot of fun.”

“I’ve been so close so many times. It’s just an unbelievable feeling,” he said. “To finally get it done on Sunday in a major is a huge monkey off my back. . . . It’s a start toward becoming a great player.”

For perspective, if this tournament were conducted by PGA Tour norms, Johnson’s simple statement to an official at the fifth hole that “I didn’t make the ball move” would have been sufficient to settle the issue. Pro golfers play by a code of honor that 99.99 percent take utterly seriously. Johnson has called penalties against himself in the past in circumstances where no one else could see what happened.

“On the fifth, I told the official what happened. [Playing partner] Lee Westwood saw it. We agreed,” Johnson said.

But the USGA takes itself so seriously that you have to leave dandruff on your blue blazer until a rules official can supervise its removal. As Johnson stood on the 12th tee, officials approached him to say that they would “review” the ruling after the round.

“There was nothing I could do about it,” Johnson said. “So I thought, ‘Let’s just focus on this next shot.’ And that’s what I did all the way to the house.”

From that moment, social media exploded with indignation. “I appreciate the support from other players. We’ve all had that happen sometime,” Johnson said. “Watching the video afterwards, I still didn’t think I made it move. They said I did.”

Then, chuckling, Johnson added, “I don’t even understand the [balance of evidence] rule. It didn’t matter at the end of the day.”

But for those final two hours, it certainly preyed on his mind — or at least the back of his mind. “Maybe a little bit,” said Johnson, reminded of a ruling after his round had ended that cost him a two-shot penalty and a chance to be in the 2010 PGA Championship playoff. Then the truth trickled out. “For sure,” he said. “Just one more thing to add to the list [of ways to lose a major], right?

“[To overcome the ruling controversy], it definitely makes it sweet. I tried my best not to look at the leader board,” Johnson said, because what good could it do him when he didn’t even know his real score. After Johnson’s last brilliant shot into the 18th — “one of the best shots I’ve ever hit” — he asked his brother, Austin, who is also his caddie, “Where do we stand?”

How about on top of the world.

One testament to the quality of Johnson’s play was the way that Oakmont, which still is a brutally exacting test of championship golf, chewed up everyone with a shot at him. Everyone near the lead collapsed, some utterly, as Johnson’s final margin of victory over Shane Lowry (76), Jim Furyk (66) and Scott Piercy (69) was a comfortable three shots.

“I credit Dustin for playing that way with that [ruling] hanging over his head,” said Lowry, who bogeyed the 14th, 15th and 16th. “I probably would have wanted to know straight away.”

The rap on Johnson has been that, beneath his super-cool exterior and languid panther pace down the fairways, he isn’t clutch in the majors, that he rattles early in Sunday rounds for the biggest titles or else his putter disintegrates late, and that complex rules issues unsettle him at vital times.

Can we call the undertakers, please? Dig several large graves all over the back nine of Oakmont and bury each of those pieces of Johnson’s past, to whatever degree they were true, deep, deeper and deepest.

Now, when it comes to Dustin Johnson, everybody knows the score. Watch out, with that monkey gone from his back, here comes golf’s 800-pound gorilla.