Doctors have removed a small tumor from the brain of Cardinal John J. O'Connor, his spokesman said today as the 79-year-old cardinal left Memorial Sloan-Kettering Cancer Center, where the tumor was diagnosed.

O'Connor will get radiation therapy "shortly and continue over the next several weeks," according to a statement released by the Archdiocese of New York. He had been hospitalized for tests since Aug. 25.

Doctors removed the tumor from the surface of O'Connor's brain, and doctors found "no other areas of the body were affected," the statement said. The archdiocese would not release any more medical information about the tumor.

The cardinal, who became spiritual leader to 2.2 million New York-area Catholics in March 1984, was brought to the hospital complaining of nausea and weakness. He walked out today to his car and later said Mass in his residence adjoining St. Patrick's Cathedral, said his spokesman, Joseph Zwilling.

In his high-profile position in New York, O'Connor has become one of the city's more prominent citizens. He has coauthored a book with ex-mayor Edward I. Koch and greeted the city's elite on the steps of the cathedral each St. Patrick's Day.

He has announced plans to retire as head of the New York archdiocese after he turns 80 on Jan. 15.