O'Connor's funeral was scheduled for Monday, May, 8, at famed St. Patrick's Cathedral.
O'Connor was the oldest active bishop in the United States and one of the country's most influential Catholics. His health began to fail after he had a brain tumor removed in August.
Although he mustered the strength to make a farewell visit to the pope in February, he missed many of the public Masses at St. Patrick's in the months following his surgery.
O'Connor, the spiritual leader of more than 2 million Roman Catholics in the New York Archdiocese, had been in poor health since the August operation. But a spokesman said the cardinal's health took a "sudden and dramatic turn for the worse" Wednesday morning.
O'Connor died at 8:05 p.m. at his Manhattan residence adjacent to St. Patrick's with his sister, other family members, clergy and colleagues at his side, said church spokesman Joe Zwilling.
He has not appeared in public since March 5, and even then he only spoke briefly to his congregation and did not read a sermon.
Last month, for the first time in the 16 years that he led the archdiocese, he was unable to watch the Easter Parade from the cathedral's steps.
Zwilling said the cardinal's condition worsened Tuesday night. O'Connor's sister, various other relatives, and fellow clergy were sitting with him-- three people at a time--in his residence behind the cathedral but the cardinal was unable to speak.
Cardinal Bernard F. Law flew to New York from Boston Wednesday to be at his side. The two men have long been friends and ideological allies. Law's spokesman, John Walsh, said: "They were made archbishop around the same time, raised to the role of cardinal at the same time, and share geographical proximity, similarities in outlook on the church, so they gravitated to one another. There was a natural chemistry there, and this relationship grew even closer during this very difficult time." Law has been one of several colleagues rumored to be a possible successor to O'Connor. Other widely mentioned possibilities include Bishop Sean O'Malley of Fall River, Mass. and Auxiliary Bishop William Murphy of Boston.
However, nobody outside the Vatican knows--or talks about--these things with any certainty. When his predecessor, Cardinal Terence Cooke died, nobody guessed the replacement would be O'Connor, who had been a bishop in Scranton, Pa., and, before that, vicar of the US Armed forces.
O'Connor was appointed to the archdiocese in 1984, and almost instantly stirred up controversy by expressing Rome's views on high-profile political issues in a far more outspoken manner than his predecessors.
Right away he publicly likened abortion to the Nazi Holocaust, denounced homosexuality as a sin, and criticized Geraldine Ferraro--then a Queens congresswoman running as the Democratic vice presidential candidate--for supporting abortion rights.
These were not uniformly popular positions to take in this archdiocese, which consists of much of New York City, its suburbs, and the Hudson Valley.
In 1987, after a trip to Jerusalem, he played a major role in persuading the Vatican to admit, and apologize for, its past record of anti-Semitism. He became close friends with New York's Jewish mayor of the time, Edward Koch. The two even wrote a book together.
He has also been friendly with the current mayor, Rudolph Giuliani, who is Catholic. But, unlike Cardinal Francis Spellman, who was called "The Powerhouse" when he ran the archdiocese from 1937 to 1967, O'Connor never sought to play a role in the city's political affairs.
Over the past decade O'Connor considerably softened the confrontational stance of his early days and became a popular cardinal, though a less influential one than he might have been in an earlier time. In Spellman's day, for instance, Irish Catholics formed a big part of the city's power elite-and every Cardinal, since the archdiocese was founded, has been of Irish descent.
However, in recent years, the Irish have faded as a unified ethnic bloc, and, apart from political considerations, a growing percentage of the city's Catholics are now Hispanics, who feel little connection to the man in St. Patrick's.
O'Connor has submitted his resignation to the Vatican for the past five years, a motion consistently ignored. It was widely believed the Pope would finally let him go last January, when O'Connor turned 80, the age when Cardinals lose their voting rights in the Vatican Council, but he was kept on the job.