THE DECISION of President Reagan to nominate Sandra Day O'Connor of Arizona for a seat on the Supreme Court is far more than the fulfillment of a campaign commitment. It marks the end of a long road for all those women who have ever practiced or aspired to practice law. Just 109 years ago, the court on which Judge O'Connor will sit if the Senate confirms this nomination upheld the power of the states to prevent women from becoming lawyers.

The vestiges of the thinking that produced that now unthinkable discrimination linger on. But the ascension of Judge O'Connor to the nation's highest court would help eliminate more of them, regardless of how she votes on constitutional questions. The fact that a woman has, at long last, been selected for one of these seats of great power will make the continuance of sexual barriers in lesser jobs more difficult to justify.

In some ways, when you think of it, it is incredible that this should have to come as such a momentous event in 1981, that it should have this aspect of novelty and "breakthrough" to it. And we hasten to suggest that it will merely compound the grotesque thinking that has created such a situation if the great legal and political powers-that-be regard a seat on the court for one female as some kind of equity. Female justices should not be considered as some one-of-a-kind token or representative or quota-filler. Mr. Reagan has helped redeem the shame of his predecessors who wouldn't dare to do what he has done. He is to be congratulated for that. Now let us hope there will always be men and women on the court and that this will come to seem ordinary.

From her record in Arizona, it appears Judge O'Connor has been a good politician, a quality lawyer and a far better than average trial and appellate judge. The kinds of cases she has handled on the state bench, naturally, bear little or no resemblance to those that routinely come before the Supreme Court. This means there are few, if any, clues in her judicial career as to how she will vote on broad constitutional questions. But this is not unusual. Rarely has the public record of any nominee laid bare his judicial philosophy, and sometimes the public record has been totally misleading as an indicator of judicial behavior.

Those who have known Judge O'Connor work over the years describe her as a conservative but not reactionary Republican and believe she is more likely to end up closer to the philosophical position of Chief Justice Burger than to that of the other Arizonan on the court, Justice Rehnquist. If that is so, the change on the court from Justice Stewart to Justice O'Connor may not alter its direction substantially.

Rarely, if ever, has a president reached so far down into the state judiciary to find a Supreme Court justice.Most of them have come from higher ranks of the judicial system, from national political positions or from the nationally known law firms. That President Reagan has gone to the second tier of a state court structure in his search for a female nominee may be less a commentary on Judge O'Connor's qualifications than on a system that, until the quite recent past, kept almost all women lawyers from reaching high places in their chosen profession. The Senate, of course, must now subject Judge O'Connor's record to the same close scrutiny it has given other nominees. We don't know how this will turn out. Our first impression of her qualifications is that the Senate will find nothing to impede her confirmation.