Pat O'Brien knew he was really tight with President Reagan when the White House sent him an egg the other day.

They wanted his autograph on it. For the Easter Egg Roll.

When you come right down to it, O'Brien is the man who made Ron Reagan a star in the first place. The veteran actor is appearing with his wife and youngest daughter in "On Golden Pond" at Manassas' Hayloft Dinner Theatre. They'll be in town for the six-week run of the comedy.

"I called Ronnie at the hospital yesterday," O'Brien said, "and he'll get back to me in a day or so."

O'Brien, already an established star, with 50 pictures behind him including "Ceiling Zero" and "The Fighting 69th," was talking to Jack Warner one day in 1939 while "Knute Rockne, All-American" was still being cast. O'Brien had the title role sewed up.

"Who you got for the Gipper?" he asked, meaning of course the great George Gipp of Notre Dame, coach Rockne's best gridiron hope. Warner shrugged. O'Brien had an idea.

"Hey, you ought to look at this kid on the lot, I think his name is Reagan, he's always fooling around with a football or baseball at lunchtime.And he's been a sports announcer. . . ."

So the kid got a screen test, and O'Brien volunteered to make the test with him, in uniform and all. The kid got the part.

"Ronnie always says I got him his start. And he signs his letters Ronnie. That's class."

Last year O'Brien and his wife Eloise had a reunion with another old pal, James Cagney, his costar in Angels With Dirty Faces" in 1938. Cagney had retired 20 years ago to raise Morgan horses on his upstate New York farm. Director Milos Forman persuaded him to come back in "ragtime," E. L. Doctorow's pseudo-historical story of turn-of-the-century New York. Cagney plays a police commissioner -- a long way from those gangster parts. O'Brien is Harry Thaw's attorney, and Eloise plays Thaw's mother.

"The funny thing is," O'Brien muttered in a talk at his rented house in Manassas yesterday, "Jimmy and I did nine pictures together -- we both started as chorus boys, you know -- and in this one we don't have a single scene together. But we had a great time for a couple of months, the four of us, with Jimmy's wife, Billie. We took a big suite outside London, saw all the plays and did the town."

Once they were taken to see "Angels With Dirty Faces" dubbed into French, and it cracked them up. Those two Irish faces talking perfect slangy French. . . .

He is a stage actor at heart. He came up that way: college theater, vaudeville, the sticks, Broadway. All his Irish cronies in Hollywood -- Cagney, Tracy, James Gleason, Barry Fitzgerald and so on -- were from the stage.

"Don't get me wrong," he said, "I'm very grateful to the movies, they made me a wealthy man. But I never liked 'em that much. I always wanted to be out there with the live audience, that crackle in the air, that magic."

He has been acting for 62 of his 81 years.

I started in the circus, you know (not counting when he was a lamb in a Christmas pageant at 5), I had a cousin Marty Hynes who was the first bareback rider to stand on four horses abreast. He was with the Buffalo Bill Wild West Show. When I was a kid I rode in the stagecoach that got attacked by Indians. I guess it bit me then."

He has played everything from football coaches (three times) to priests (most notably in "The Fighting 69th" and "Fighting Father Dunne"), from heavies to a relentless cope in "Some Like It Hot." Recently he played the Stage Manager in "Our Town" with a New Hampshire accent. But there are two roles he would have loved and never got: Ulysses S. Grant and Cyrano. "Maybe I can coach some younger man in them," he muttered wistfully. The play about Grant hasn't even be been written yet.

He can't understand what happened to that fine movie genre, the bio-pic. A whole generation of Americans were raised on Don Ameche's Graham Bell, Spencer Tracy's Edison, Paul Muni's Pasteur, Zola and Juarez and other classics. He misses the great days when Hollywood still found it possible to make a major picture without a lot of skin and ketchup.

He can't stand television.

"So you don't see your old pictures, then?"

The gray eyes popped. "Do I! Every chance I get!" Two-beat pause. "I turn the dial and watch my hairline recede."

You forget what an old pro Pat O'Brien is. The hard school of vaudeville made him a master of timing, a cool hand who simply cannot be ruffled onstage, who will calmly invite a heckling drunk to take over the mike at a nightclub gig, letting the audience boo the guy away.

He thinks his current role, a crusty 82-year-old man saved from sinking into senility by his lively grandson, is the nicest one he ever had. People come up to him all the time to tell him he reminds them of their father or grandfather. He might take the play to Ireland when he's done with his American dates.

"Maurice Evans is doing it in Florida. Can you believe it? With an English accent?"

Trim at 168 pounds, O'brien eats a giant breakfast (Grape-nuts and bananas, sourdough toast, three poached eggs and a Coca Cola), no lunch a light supper. He drinks only wine and beer these days, rarely after supper, "and never, ever, when I'm working." He smokes about one cigar a day.

"I used to like a drink of the holy water now and then (that is, of course, the dew, the poteen, usquebaugh, the Irishman's bottled friend), but I found out I didn't need it."

O'Brien met Eloise McGovern before most of use were born, when they were both in a road production of "Broadway." He saw her coming through the stage door, admired her legs, patted her -- and got hit with a snappy left hook. For several years he courted her, mostly by letter because they were on the road, but finally he said, "Let's get married or something," and she said, "Let's get married -- or nothing!" That was 1931.

(So you heard the line before. So it's still a good story.)

There are two sons, two daughters, including Brigid, the actress, who plays their daughter in the comedy, and six grandchildren.

"God has been good to me," he said.

Certainly something has been working for him. He got his big break when Howard Hughes signed him up to film "The Front Page" in 1931. He had been appearing in the stage version a year ealier. Halfway through rehearsals he was asked how he handled some stage business in his starring role of Hildy Johnson.

"Don't ask me," he replied. "I didn't play Hildy. I played the managing editor."

"But on Broadway -- "

"It wasn't on Broadway, either. It was in Cleveland."

"But how come -- "

"You never asked."

It was all uphill from there.