Starting with ''Francis'' in 1950, Mr. O'Connor made a lot of money playing opposite a mule. The film proved so popular, it was followed by ''Francis Goes to the Races'' (1951), ''Francis Goes to West Point'' (1952), ''Francis Covers the Big Town'' (1953), ''Francis Joins the Wacs'' (1954) and ''Francis in the Navy'' (1955).
Hollywood also turned out ''Francis in the Haunted House'' (1956), but by then, Mr. O'Connor had decided that a continued association with a mule was not a good thing for his career, and the human lead went to Mickey Rooney.
Mr. O'Connor also did a great deal of television, starring in the 1950's in ''Colgate Comedy Hour.'' John Crosby, the television critic for The New York Herald Tribune, saw him perform with Jimmy Durante on ''Texaco Star Theater'' and called Mr. O'Connor ''one of the greatest all-around talents in show business.''
But the golden age of the Hollywood musical was over, and as Mr. O'Connor aged and thickened, he no longer looked perfect for juvenile roles. His career faded, not to be revived by his performance in the title role of ''The Buster Keaton Story'' in 1957.
He appeared in few movies after that. His brief appearance in ''Ragtime'' in 1981 was the first film he had done in 16 years.
Somewhere along the line, Mr. O'Connor developed a serious drinking problem. He said he started drinking while in the service in 1944, and later on, he said, ''instead of coming home and having one or two drinks, I'd have one or two bottles.'' Work was even harder to come by.
He stopped drinking in 1979, but he developed heart trouble and began to use nitroglycerin pills before performances so that he would have the stamina to complete them. He underwent successful quadruple-bypass surgery in 1990. After that he began to watch his diet. He worked out three times a week, and, in 1992, told a reporter that ''sobriety has been my savior.''
Mr. O'Connor came to Broadway in 1983 in a revival of ''Show Boat,'' in which he played the boat's proprietor, Cap'n Andy. In his review in The Times, Frank Rich wrote of Mr. O'Connor: ''He gets his laughs, by hook and crook, and contributes a delightful (if brief) display of his old razzmatazz tap style in Act II.''
In 1997, at the age of 71, Mr. O'Connor made a rare movie appearance in ''Out to Sea,'' a comedy starring Walter Matthau and Jack Lemmon. With him in that small part was Gloria De Haven, who had starred with him in ''Yes Sir, That's My Baby'' in 1949.
His 1944 marriage to Gwendolyn Carter ended in divorce 10 years later. They had a daughter, Donna. His second marriage, to Gloria Noble in 1956, lasted for the rest of his life. They had three children, Alicia, Donald Frederick and Kevin.
Mr. O'Connor prided himself on being forward-looking, and he was not inclined to dwell on his past.
''I'm no longer a superstar,'' he said in 1992. ''Now I'm working on being a quasar, because stars wear out. Quasars go on forever.''
He said that as a young man, he had always been afraid of failure but that stopped as he grew older.
''Now I look for the parts where I die and they talk about me for the rest of the movie,'' he said.