By naming Arizona Judge Sandra O'Connor to the U.S. Supreme Court, President Reagan nominated a first-rate justice who will add to the court more than a profound theoretical knowledge of the law. A judge of the Arizona Court of Appeals, Mrs. O'Connor is a respected legal scholar. During a distinguished political and legal career in Arizona, however, she also has demonstrated a deep sensitivity to the practical aspects of law. She is no ivory tower theorist who regards the law as the exclusive realm of a special priesthood of attorneys, something apart from the ordinary people who must live by that law. She developed that outlook not only through a logic generated by a superior intellect but also because she has been so much a part of the lawmaking process and so involved in community affairs. She is the first and only woman to hold a high leadership position in the Legislature, selected as Senate majority leader in 1973, not out of any zeal on the part of her colleagues to set an example for equal opportunity but rather because her constant pursuit of excellence earned the admiration of other senators. Because Justice-select O'Connor happens to be a woman, President Reagan gets credit for keeping a campaign promise and may gain some political leverage m the women's movement. Be that as it may, the U.S. Supreme Court and the nation stand to benefit from the appointment.