The court's newest justice quickly makes her mark

A year ago June 7, President Reagan made history by announcing his selection of the first woman member of the US Supreme Court - Sandra Day O'Connor.

Now, as Justice O'Connor completes her first term, it is clear that she will make her mark as a justice, not just as the first female justice.

Able, forthright, and articulate, O'Connor quickly mastered the court's procedures and protocol, along with the enormous body of legal material which flows in and out of the justices' chambers.

Although she was its newest and youngest member - at age 52 more than twenty years the junior of most of her colleagues - neither fact seemed to intimidate O'Connor. She demonstrated that she knew her own mind and how to speak it. In her first term, she wrote several major opinions for the court, penned some equally important dissenting views, and did not hesitate to criticize her colleagues for sloppy work.

One term is too narrow a base upon which to build an accurate assessment of a justice. But certain characteristics of O'Connor's performance are worth noting.

She most often voted on the conservative side of an issue, joining Chief Justice Warren E. Burger and her Stanford law school classmate William H. Rehnquist. And just as significant as her vote with this conservative core - which was often joined by Lewis F. Powell Jr. and Byron R. White or Harry A. Blackmun to form a majority - was the persuasive and articulate voice she provided for their views.

But O'Connor did not always take the conservative side. Among the most notable of her disagreements with Burger and Rehnquist came over questions of sex discrimination. Over their dissent, she voted with the majority to hold Mississippi University for Women in violation of the Constitution for refusing to admit a man to its nursing program. Earlier, she was part of a majority that held that the federal government is authorized to monitor the employment practices of schools and school districts that receive federal aid.

When Chief Justice Burger wrote an opionion that O'Connor considered unsatisfactory, she said so - criticizing it for establishing ''an uncertain jurisprudence.''

Sex discrimination, criminal law, and state powers drew O'Connor's most forcefully expressed views.

When the court reaffimed the exclusionary rule, which bars use of illegally obtained evidence in court, she dissented. When they further limited the crimes for which the states can impose a death penalty, O'Connor, dissenting, made clear that she would not restrict that sentence only to persons who actually killed others, but would allow its use for those who assisted in such crimes.

When the court obligated states to educate illegal alien children, O'Connor - who grew up in a state well-acquainted with the alien problem - dissented. When it struck down Washington State's anti-busing law, she objected. But when it held that presidents were completely immune from damage suits based on their official actions, she agreed.

In a Mississippi school case, O'Connor, speaking for the court, delcared that laws which treat men and women differently will be upheld only if there is an ''exceedingly persuasive justification'' for the different treatment of the sexes. Women's rights groups welcomed this announcement. Not only did it make plain O'Connor's position on the question of sex discrimination, but it toughened the standard against which the court has tested such laws in recent rulings.

As might be expected of someone with careers in all three branches of state government - serving as an assistant state attorney general, a legislator, and state judge - O'Connor is a strong advocate for state's rights and state's power.

She wrote three opinions in which the court curtailed the power of federal judges to review criminal convictions obtained in state courts. She concurred in most other decisions that limited federal power and enhanced that of the state's.

O'Connor's strongest statement in defense of state's prerogatives came in dissent as the court upheld a law that directed states to encourage more efficient use of electricity by utilities and their customers. O'Connor argued that Congress had reached too far into the business of the state. Giving new vigor to the Tenth Amendment - which reserves certain powers to the states and the people, and which has been moribund since New Deal days - she left no doubt that states have a strong new voice in the nation's highest court.