The signs around proud old Oakmont Country Club say, "No Trespassing." Now, a postscript can be added: "And no more 63s."

For a decade, Oakmont and the USGA have filed away the embarrassing memory of Johnny Miller's final-round 63 in 1973 as an item to be dealt with in some future agenda. In today's first round of the U.S. Open, that bill came due.

This was the day that 156 golfers paid for Miller's impertinence long ago. The USGA believes, as a first principle, in a vaporous postulate that it calls "the integrity of par." That means when you play in the U.S. Open, you whimper. You don't shoot 63 to cop the silver bowl. Especially not at hallowed Oakmont, the only course the USGA has deemed fit to host six of its Open shindigs.

So it came to pass that the rough here was allowed to grow. And grow. Until a bulldozer with a full shoulder turn couldn't get a golf ball out of it. And those legendary Oakmont greens were encouraged to become firm and fast.

As a result, only three men--Seve Ballesteros, John Mahaffey and Bob Murphy--were able to break 70 this muggy day, sweating their way to two-under-par 69. One other fellow, veteran Bruce Devlin, managed a 70, and Bobby Wadkins, Lou Graham and D.A. Weibring equaled par. Other than those lucky seven in the red, everybody else was in bleak black figures.

In all, only two dozen could score 73 or better. Tom Watson (72), Ray Floyd (72), Lanny Wadkins (72), Hale Irwin (72), Jack Nicklaus (73) and Arnold Palmer (74) practically rejoiced after their over-par rounds. The only way to shoot 63 today was to quit after 15 holes.

The stroke average for the suffering field was 77.1, with 44 players over 80. You have to go back to Pebble Beach in 1972 to find higher Open scores. And those numbers were shot in a chilly gale; the weather here was warm and windless. Fittingly, Johnny Miller started with a double bogey and shot 78.

Other famous names joined in the misery: Andy Bean (76), Bobby Clampett (76), Craig Stadler (76), Jerry Pate (78) and Fuzzy Zoeller (79). George Burns took a 10 on the par-4 18th for 83, then refused to sign his score card, even when asked to do so, and was disqualified. Fellows like Calvin Peete, Tom Kite, Tom Weiskopf and Bruce Lietzke shot 75, yet found themselves still in the midst of the hunt.

As befit the occasion, the best golfers in the world gathered on one knee like penitants to admit their manifold sins and weaknesses. Oh, Oakmont, Oh, USGA, we beseech you to hear us.

"This is absolutely the toughest course we have ever played, bar none," said coleader Murphy, "and I've played some public golf courses that were pretty neat."

"It's totally intimidating. It is the most difficult course I've ever played and that's with the weather absolutely perfect . . . I told my wife that if I could break 80, I'd achieved something," said Devlin.

"No, I don't think it's fair. If you miss the fairway by six feet, it's all you can do to hack it out 30 yards in front of yourself. There's no talent in playing out of nine inches of rough . . . If you get too aggressive, this course will eat you alive. This will be a week for conservatives.

"It's taken the chipping out of the game. Around the greens, all you can do is sand-blast it out of the long rough."

Having had its fun, the USGA relented after nightfall, issuing an amusing announcment that since the rough was "in excess of the prescribed five inches . . . the USGA has decided to cut the rough in certain areas" before Friday's play.

Of the four men under par, two were of particular interest--Mahaffey, who won the 1978 PGA here, and Ballesteros, who won last week at Westchester and who is, once more, deep into his I'm-not-appreciated-in the-U.S. routine.

"I made about everything. Some of my one-putts were just amazing . . . To shoot 69, hittin' it like I'm hittin' it, is phenomenal . . . I was lucky I didn't shoot 75 or 78 from some of the places I drove it," said Mahaffey, whose whole round was one constant adventure. He had six birdies--at Nos. 2, 5, 6, 9, 13 and 14--plus three saves of par (two on putts of 20 feet), but also drove the ball so wildly that he made four bogeys.

"This is the way a U.S. Open ought to be . . . Whoever said it was supposed to be fair?" added Mahaffey, who insisted, "I'm going to keep on hitting my driver. If I'm gonna be in the rough, I might as well be long."

Ballesteros, like most of the long hitters here, is adopting the opposite strategy. "Last week I hit 23 times my one-iron," said the flamboyant and gifted Spaniard, who used irons off the tee on 11 of 14 driving holes, once even using a five-iron on the 343-yard, par-4 second hole. "I wish I could hit my driver more, but the USGA just makes the course to make me play that way."

Perhaps Ballesteros flatters himself that the sudden and drastic tightening of driving areas, once you get more than 240 yards from the tee, is a conscious anti-Seve tactic. At any rate, Ballesteros harped a dozen times this afternoon on his pet beef--that Americans think of him as a wild hitter who is lucky, rather than as a marvelously talented player.

"In Europe, they don't tell me I am lucky, so I don't tell them I am lucky," said Ballesteros pointedly. "I am just lucky in the United States."

The great Ballesteros birdied the second, sixth and 13th holes and eagled the 480-yard ninth with a drive, a four-iron shot and an eight-foot putt. His bogeys at the first and third came after he missed the fairway with irons, and he three-putted the 18th green from 25 feet to avoid being the leader alone.

Of the half-dozen imposing names lurking at 72 and 73, Watson and Nicklaus seem pleased with their games and still anticipate a repeat of their duel last June at Pebble Beach.

"I'd have to say I'm pretty happy," said defending champion Watson, who, before he teed off, said he was playing "like a four-handicapper." Watson had only one birdie but kept his pluck. "I didn't play well, but I scored well . . . which is what I did last year. I just forced myself to put the ball in play."

Once, in the bad old days when the Open seemed to have him jinxed, Watson talked constantly about the mystical intricacies of his swing and seemed to be searching for excuses. Now, like most great champions, he recognizes that when the Open rolls around, you take the game you've got and, if you think you're the top dog in your sport, you just make it work.

After a steady but drab one-birdie round, Nicklaus seemed to employing an old Ben Hogan gambit. When everybody says the course is impossible, you say you don't think it's so hard. Gets 'em confused.

"I didn't think it was that tough," said Nicklaus of Oakmont, where he won in 1962 his first of four Opens. "I thought I made it tough. I tossed away about four shots . . . I didn't get much in the hole. I'm surprised the scores aren't lower. Gary Player (73) said he thought nobody would break 290 this week. I told him I thought you'd have to break 280 to win."

Asked if he felt he was playing well enough to win, Nicklaus, who usually hems and haws on such points, said, "Yes, I feel that way . . . While I have, at least in my own mind, the ability to play, I want to make the most of it."