John F. O'Donnell, 85, a Lawyer And Advocate for Union Workers

John F. O'Donnell, a leading labor lawyer who represented the national postal and transport unions and played a central role in New York City's transit strikes, died yesterday at Northern Westchester Hospital in Mount Kisco, N.Y. He was 85.

He died of cancer, his family said.

Mr. O'Donnell lived in Katonah, N.Y. He had been the general counsel for the Transport Workers Union of America since 1948 and was also the chief lawyer for the American Postal Workers.

He was at the center of many tense transit negotiations, including the 11-day strike in 1980. Among his early victories was getting $1.1 million in back pay for workers on the old Third Avenue Railway in 1950 and helping win a five-day, 40-hour work week in a 28-day strike of eight bus lines in 1953.

"Some people specialize in problems, but he always was looking for the solutions," said Theodore W. Kheel, the labor mediator. Mr. O'Donnell was known among labor insiders for his gargantuan cigars, Irish brogue, meticulous preparation and articulate presentation. Career Developed by Chance

His career developed by chance when he became an aide and friend to City Councilman Michael J. Quill in the 1930's. Mr. Quill helped found the transport workers union and went on to became its fiery leader.

Mr. O'Donnell was born in Donegal, Ireland. A supporter of the Irish Republican Army, he was caught with a pistol by British authorities, and to avoid further trouble, he moved to New York when he was about 20.

He worked varied jobs, which included elevator operator, grocery store clerk, teacher of delinquents and a writer of newspaper editorials for The Irish Echo.

After undergraduate study at City College, he earned a law degree at Fordham University in 1937, attending night classes. Formed a Law Partnership

In 1948 he and Asher Schwartz formed a law partnership in Manhattan and later formed an affiliated law firm in Washington specializing in labor law and union clients.

His clients in recent years included Local 3 of the International Brotherhood of Electrical Workers Utilities Division, Communications Workers of America Locals 1101 and 1105, and Local 1S of the Retail, Wholesale and Department Store Workers Union.

He is survived by his wife of 57 years, the former Gwendolyn Large; a son, John Jr., also known as Sean, of White Plains; three daughters, Patricia O'Donnell of Katonah, Cathleen Paprocki of Chicago and Eileen Sheehan of Charleston, S.C.; a brother, Louis, of Donegal; two sisters, Lily Henry of Dublin, and Mary Loranger of Grosse Pointe, Mich.; eight grandchildren, and four great-grandchildren.