For Larry Nelson and Tom Watson, this Sunday at the U.S. Open was full of glory and frustration, birdies and lightning, and, finally, hours of thunder and exasperating torrential rain.

What was decided this afternoon, before thunderstorms suspended play late in the fourth round, was that Nelson and Watson had made this Open their own private competition. Thanks to their spectacular play at Oakmont Country Club, they left the rest of what had been a bunched field well in their wake.

When the downpour came, interrupting a final Open round for the first time in history, both Nelson and Watson stood at four under par, three strokes ahead of Gil Morgan. Calvin Peete and Seve Ballesteros, who were one over par both for the day and the tournament, were the only other players in sight. Hal Sutton was the leader in the clubhouse at 71--287, three over par.

What has not been decided is which of these men, who are not friends, will be the champion of the 83rd U.S. Open. When play is resumed on Monday at 10 a.m., Nelson will be on the 16th tee, while Watson, playing in the last twosome, will have a difficult downhill 35-foot birdie putt on the 14th green.

Should a playoff be necessary, it would go 18 holes and start at noon, though ABC-TV (WJLA-7) has decided to replay Monday's action from 4 to 5:30 p.m.

When the storm siren sounded, Nelson had completed 15 almost-flawless holes, sinking five birdie putts and making but one three-putt bogey. Watson, who made six birdies while shooting a 31 to tie the course record on the front nine, was three under par for the day when lightning interrupted his quest to become the fourth player since World War I to win back-to-back Opens.

Both Nelson and Watson had little barbs for each other this evening, tiny golfing messages to sleep upon.

"On any Open course, it's always an advantage to have less holes to play," said Nelson, who has played his last 33 holes in 10 under par. "The 15th (a 453-yard par 4) may be the hardest hole out there. I've already played it and Tom hasn't, so I'd say I have a small advantage."

"If I can finish four under, I'll win the tournament," said Watson, unequivocally. The only possible logical interpretation of this remark is that Watson believes that Nelson will play the final three holes over par and thus hand him the tournament. Asked pointedly if he had really meant to say what he said, Watson replied, "Four under is not a bad bet . . . I probably won't win this tournament if I make more than one more mistake. I don't think one mistake will beat me, but two mistakes and I might shoot myself out of it."

As for the longshot chances of Morgan, who still has to play the 17th and 18th holes, Watson said, "Stranger things have happened. If he can birdie the 17th (a 322-yard par 4) and get to two under, he might still win."

Watson's desire to win this Open is enormous, and not just because he could be the first player since Ben Hogan in 1950-51 to repeat as titleholder.

Watson has said all week that he has "a compulsion" to atone for losing the 1978 PGA here when he had a five-shot lead at the turn, then eventually lost in a three-way playoff. Something similar happened today. Watson held a three-shot lead over Nelson after he birdied Nos. 8 and 9. However, Watson drove in the rough and bogeyed the 10th and 12th holes. When Nelson knocked a nine-iron shot a foot from the hole at the 14th for a birdie to reach four under, Watson's seemingly safe Oakmont lead had evaporated once more.

It is an interesting, though perhaps peripheral sidelight, that more than a little friction has existed between Watson and Nelson recently, due to the golf world's considerable range of backroom politics. Watson was the instigator of a strongly worded letter of protest and petition to the PGA Tour Tournament Policy Board just two weeks ago. In essence, the letter called into question the judgment of the Board in setting directions for the game and, by implication, challenged the leadership of Commissioner Deane Beman.

Though Jack Nicklaus and Arnold Palmer took credit for originating the letter, Watson--one of a dozen famous signers--is widely considered to have been the behind-the-scenes force. Nelson is one of the four player directors of the Policy Board to whom the critical letter was directed; he also is one of Beman's strongest supporters. Nelson and Watson have no personal quarrel, but they are on diametrically opposite sides of many PGA Tour policy issues.

This aborted round will be remembered as the day that four significant, proven players went to the first tee within one stroke of the lead and met dramatically opposite fates. Nelson and Watson prospered while Peete and Ballesteros floundered.

Peete, trying to become the first black to win a major golf title, was shaky from the start; he missed the first two greens because of poor iron shots, made bogeys and disappeared permanently from contention. At least Peete righted himself thereafter. Ray Floyd, who started the day two shots behind third-round leaders Watson and Ballesteros, came completely apart. Floyd was seven over for the day through 16 holes.

Ballesteros could hardly be faulted for his mortal play. He was blown away with a front-nine knockout punch that was an almost perfect duplicate of the one he dealt Watson at this spring's Masters. Then, Ballesteros and Watson played together, both starting the day on the heels of coleaders Craig Stadler and Floyd. Ballesteros started birdie-eagle-par-birdie and shot 31 on the front.

Today, it was Watson who sank birdie putts of 5, 28, 4, 6, 6 and 4 feet for birdies at the second, third, fourth, sixth, eighth and ninth holes. Only a bogey at the seventh, when Watson hashed the simplest of fluffy 20-foot sand shots, kept him from shooting a 30. On the par-3 sixth and eighth--201- and 228-yard beasts--Watson hit glorious shots. His five-iron shot at the sixth missed the stick by three inches, then ended up two paces behind the hole, while his towering, drawing four-wood at the eighth ended up in the shadow of the flag.

"Basically, I played very well, just hit a lot of good shots," said Watson, who seemed a bit dispirited after play was called. "But I hit a couple in the rough at 10 and 12, and you get punished for that here . . . Being out in front after nine holes, doesn't win the tournament . . . It happened to me here in '78 and it happened again today."

While Watson's play seemed to flag after his torrid start, Nelson just seemed to get sharper. After starting this Open 75-73, he shot a tournament-best 65 on Saturday and now has 20 birdies at Oakmont to Watson's 15.

Many a fan will think that Nelson's best chance at an Open title is now gone with the rain. He'll wake up and realize that he's a nag who's just turned for the homestretch in the Kentucky Derby and he's facing a thoroughbred who's been there a dozen times. After all, at age 35 Nelson has only six PGA Tour victories and one major (the '81 PGA), while Watson has 28 titles, plus four British Open crowns and seven majors in all.

A victory here would give him three of the last five major titles, and five majors in the '80s.

These two are from different golfing classes. Watson is a champion with credentials that put him in the same class with Hogan and Arnold Palmer. Nelson is a determined plodder who has hot streaks, but also suffers through enormous droughts, like a year in putting Hades from which he has just emerged.

Despite this, it was Nelson who seemed chipper and upbeat this evening. On the course, he did little dances to aid his putts and, in general, seemed to be having fun in a setting where most men crumble.

So at peace is Nelson that, when a huge lightning jag lit the sky just as he was about to strike a 10-foot birdie attempt at the 15th, he barely reacted at all. "The only thing that worried me was that if it hit me, I wouldn't be able to finish the hole," joshed Nelson. "I knew where I was going if it hit me."

That puts him ahead of most folks.