WASHING TON - Supreme Court Justice Sandra Day O'Connor hit the Washington party circuit hard, e•en 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 canapes and cocktails. While she is most often in the majority in the court's conservative opinions, and has a reputation for being I • 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. 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. Only the president, vice president or the . ~hief justice rate higher. 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 freshman justice, she should uphold the court's stuffy demeanor. • . Perhaps