Sandra D. O'Connor became an associate justice of the Supreme Court yesterday, the first woman in U.S. history to bear that title.

Chief Justice Warren E. Burger administered the constitutional oath of office at 2:16 p.m. before about 400 dignitaries and friends of the O'Connor family who packed the mammoth Supreme Court chamber.

"Justice O'Connor, welcome," Burger said simply. Then, after being helped into her black judge's robe, O'Connor took the chair assigned to her by seniority, the one on the end of the bench to Burger's extreme left.

Sitting next to her was her Stanford law school classmate, Justice William H. Rehnquist.

From the raised bench, where no woman has sat in the 191-year history of the Supreme Court, she smiled down on President Reagan just below her.

O'Connor is the 102nd justice and the first appointment to the court by Reagan. At 51, O'Connor is the youngest member of the court, which has five justices over 70.

Her first public appearance at the court came at noon yesterday. She and Burger, his snow-white hair glistening in the sun, descended the front steps to pose for pictures. Burger clutched her arm and commented to reporters that "You've never seen me with a better-looking justice."

O'Connor's husband, three sons and mother and father joined them on the court plaza for the pictures. She shouted to a friend to take pictures of the photographers for her scrapbook.

She then went inside to take her first oath of the day, the "judicial oath," which calls upon her to "do equal right to the poor and to the rich," in the privacy of Burger's chambers.

Shortly after 2 p.m., Supreme Court chief clerk Alexander Stevas escorted her into the silent but jammed Supreme Court chamber, where she sat alone in a chair used by Chief Justice John Marshall in the 19th century.

Attorney General William French Smith presented her to the eight other justices. Placing her left hand on a Bible and raising her right hand, she took the constitutional oath, swearing that she will "well and faithfully discharge the duties of the office on which I am about to enter. So help me God."