Supreme Court nominee Sandra D. O'Connor, facing her first day of confirmation hearing questioning, said she made a mistake 11 years ago when she voted in the Arizona legislature to decriminalize abortion.

It was her one important concession, in a day filled with questions about abortion, to the conservatives who have challenged her nomination. But she declined repeatedly to go any further and say how she would rule on that or any other issue on the high court.

She assured the Republican-dominated Senate Judiciary Committee, however, that she believes in a limited role for the federal judiciary in American life.

"I do not believe it is the function of the judiciary to step in and change the law because the times have changed," she said. "I do well understand the difference between legislating and judging.

"As a judge," she said, "it is not my function to develop public policy."

The opening of the O'Connor hearings, as senator after senator noted yesterday, was a historic event: the first time the Senate has ever considered a woman for the Supreme Court. Easy Senate confirmation is expected within the next three weeks.

It was also the first time since her nomination that O'Connor has explained her record and her views publicly, though she began the hearings by telling the senators that it would be improper for her to be too specific in answer to questions about specific issues she might confront on the court.

She did get specific about her actions as an Arizona state senator that have produced the only significant opposition to her nomination. She conceded, as she apparently had not in previous conversations with Reagan administration officials, that she cast a 1970 committee vote (prior to the 1973 Supreme Court ruling legalizing abortion) in favor of decriminalizing abortion. She said she had to consult old newspaper reports to refresh her memory.

"At that time I believed some change in the statute was appropriate," O'Connor said in response to a question from Judiciary Chairman Strom Thurmond (R-S.C.).

"I would say that my own knowledge and awareness of the issues and concerns have increased since those days," she said. "It was not the subject of a great deal of public attention or concern at the time.

"I would not have voted, I think, Mr. Chairman, for a simple repealer thereafter." The bill failed in the Arizona Legislature.

She defended her position on other votes challenged by the anti-abortion lobby. A family planning bill she co-sponsored, she said, was designed to provide contraception, not abortion, to Arizonans.

She voted against a 1974 legislative resolution supporting an anti-abortion amendment to the U.S. Constitution because "I was not sure at that time we had given it a proper amount of reflection."

And she opposed a rider to an appropriations bill barring abortions at a University of Arizona hospital in 1974 because she thought a rider was a procedural violation of the state's constitution.

She did vote, she said, to restrict abortion funding for poor people in Arizona, though ultimately the state decided to avoid Medicaid entirely.

O'Connor said she has an "abhorrence of abortion. It is a practice in which I would not have engaged. I'm sensitive to other views, but this comes from my own upbringing . . . my sense of how I should lead my own life. It's just an outgrowth of what I am."

But "my personal views and beliefs have no place in the resolution of any issue," she told Sen. John P. East (R-N.C.), who was pressing her the hardest on the issue. And she declined to express her views on the Supreme Court's 1973 ruling or even hint at how she might rule on future abortion issues.

She did make a point, at the beginning of the hearing, to introduce her husband and three sons, and endorse marriage and the family as "mankind's basic unit of society, the hope of the world and the strength of our country . . . . "

That reluctance to comment, traditional for Supreme Court nominees, dominated all her responses to substantive issues. In response to questions about school busing, she simply recited the history of Supreme Court rulings. Asked for her views on proposals to strip the court of jurisidiction, she said "there were many views" about it.

She was questioned closely about a law review article she recently wrote suggesting that the nation's most used civil rights law, the Civil Rights Act of 1871, was being overused. She said she had not meant to suggest that victims of constitutional violations should be denied access to the courts, just that the state courts, as opposed to the federal courts, could handle more of these complaints.

On discrimination against women, O'Connor had more to say. "I don't know that I've experienced much of it," she said. "My only disappointment came when I graduated from Stanford Law School near the top of her class and I was not successful in finding employment with any of the major firms."

Discrimination against women, particularly disparities in pay, "has always been a matter of concern," she said.

"How do you want to be remembered?" Sen. Max Baucus (D-Mont) asked her.

"Here lies a good judge," O'Connor responded. "But I'm sure I would be remembered as the first woman" on the Supreme Court.

Her testimony is to continue today.