Supreme Court Justice Sandra Day O'Connor hit the Washington party circuit hard even before she received the Senate's blessing to sit on the high bench 18 months ago. Since then she has been as visible on the social scene as the ubiquitous purple Ridgewell's truck.

While she is most often in the majority in the court's conservative opinions, and has a reputation for being well-prepared for court sessions, O'Connor has become a dissenter from its tradition of keeping a low profile.

Supreme Court justices are a reserved lot, rarely keen on after-hours socializing with Washington's ruling circles. Appointed for life, they are about as close as this town gets to royalty. Which means they have no need to answer to anyone but themselves and, for the most part, have chosen isolation instead of the well-worn Washington paths from party to party.

"She is very visible socially," says retired justice Potter Stewart, who was quite visible himself during his years on the court. "She's attractive and likable and, of course, the first woman on the Supreme Court. People want to see her."

Indeed, O'Connor and her husband, John, are considered a big draw by the big guns in the Washington party whirl, next only to the president, vice president or the chief justice. She is a novelty: the court's first woman, young (53), a good dancer and the picture of self-containment, obviously confident enough not to worry that as a groundbreaking freshman she should conspicuously uphold the court's stuffy demeanor.

Perhaps because they've been hard to get, until O'Connor, justices on the whole are considered permanent "A" list material in Washington's ever-changing social structure.

"The chief justice--that's where the big push is," says Gretchen Poston, former White House social secretary and partner in the party-planning firm, Washington Whirl-Around. But Chief Justice Warren Burger has a reputation for choosing only one event a week to attend out of many invitations. And he rarely goes the night before oral arguments, notes a court spokesman.

"It just could be that the other justices don't go out because they are older and the pressures and the workload are so heavy and continual," says Barbara MacGregor, wife of attorney Clark MacGregor and a friend of the O'Connors. "The O'Connors are young and they are anxious to get to know people here."

"In terms of the parties I have, the O'Connors are the ones that show up the most consistently," says Mary Pettus, a professional Washington events organizer. "There is a whole kind of cachet the court has that is transmitted to the justices. So they are always invited."

"I don't think you expect them to go out much because they are not in the glad-handing business like politicians. Justices are expected to be above all that," says Sandra Westin, head of public relations for the National Portrait Gallery, where the O'Connors have attended two dances.

O'Connor gets more than 1,200 letters a week, more mail than any of the other justices. A spokesman for the court, however, said O'Connor does not know how many social invitations she receives a week.

They have been to clubby dinners, gala benefits, receptions for kings and testaments to cowboys. And a party for Donny Osmond. At the Wolf Trap Ball once, they even won the door prize: round-trip air fare for two to Morocco.

Here's a two-month sampling of what the O'Connors have attended recently:

* A small White House dinner for England's Princess Alexandra.

* A reception at the Embassy of Ecuador for President Osvaldo Hurtado, where O'Connor told guests that she had walked over from her home.

* The Gridiron Club's annual white-tie roast of Washington officialdom, where she sat at the head table.

* The Library of Congress' opening night party for "The American Cowboy" exhibit.

* A State Department reception for sculptor Louise Nevelson.

* The Silver Screen Ball at the National Portrait Gallery for the exhibit "Hollywood Portrait Photographers, 1925-45."

* A birthday dinner-dance for former congressman Robert McClory at the Fort McNair officer's club.

* An F Street Club party for Daniel Terra, U.S. ambassador-at-large for culture.

* A dinner dance given by Tazewell and Jan Shepard for writer-composer Richard Adler.

* Just a week ago, O'Connor, in flowing purple silk, charmed guests at the Corcoran Gallery Ball, dancing with a string of partners while most of her court colleagues were at an intimate dinner celebrating Justice William Brennan's recent marriage.

Court observers are quick to point out that O'Connor's dynamic social schedule has not detracted from her performance on the bench. She is, they say, extraordinarily well prepared during oral arguments. Her questions are tough and provocative, and her opinions detailed and meticulous. "She is just about the best prepared of all of them," says one.

The O'Connors are considered lively and enthusiastic guests. He's a chatty conversationalist, with a dry irreverent wit. She is known to be an intense listener, remarkable for remembering names and always offering a specific personal comment to whomever she is speaking with.

And they both are rather fast and fancy dancers, with a ballroom routine, one friend says, that they developed during the early years of their marriage.

"They're fabulous dancers," says longtime friend Juliet Folger. "John is always complaining that I'm not fast enough for him." John O'Connor once confided to a dinner partner that he had only discovered three Washington women who could keep pace with him on the dance floor.

John O'Connor told the same dinner partner that after a year here, they usually know what invitations to accept. If in doubt, he said, they ask friends for guidance. Only once, John O'Connor noted on the same occasion, did the O'Connors believe they were exploited because of his wife's title.

"To an extent, I suppose, you don't want to be used," says Stewart. "When I first came here I realized that anyone who was a justice was invited to many things. They were inviting the title. I could have been a police dog."

In a 1972 speech before the Virginia Bar Association, Justice Lewis Powell said flatly that he did not "enjoy the limitations which the ethics of our profession impose upon a judge in terms of non-participation in so many interesting phases of life--social, political and business."

Once, O'Connor phoned a Washington hostess herself to ask about the nature of the party. "It was a party for Wolf Trap," says Mary Jo Campbell, "and she said, 'I really want to go to your party, but you have to assure me it isn't a fund-raiser. We're not allowed to attend fund-raisers.' I told her it was merely a gathering of friends for Wolf Trap and she came."

The O'Connors have adopted inside Washington as their own. They live in affluent Northwest and asked that their address be dropped from the social Green Book this year for security reasons. In addition to their evening commitments, they spend time playing tennis and golf with their friends.

"My husband plays golf with them a lot, and she's as good as the men," says socialite Jan Shepard. "She actually outdrives the men sometimes."

The question, to veteran partygoers who have long since given up equating fun with Washington's official social life, is: Why do the O'Connors go at all? The answer could be that Sandra Day and John O'Connor have discovered that they don't have to do any business or politicking at parties. They don't give press interviews, and everyone knows it would be unseemly to discuss court business with them. They just might be the only couple from Washington's highest echelon who can go out merely to have a good time.