The Democratic Party strategist who organized the successful senatorial and Presidential campaigns of John F. Kennedy. Mr. O'Brien was twice named chairman of the Democratic National Committee and while he was chairman the committee's offices in the Watergate office and apartment complex in Washington were burglarized, resulting in scandals that led to President Richard M. Nixon's resignation on Aug. 9, 1974. Mr. O'Brien, who served as the Congressional liaison representative for President Kennedy and President Lyndon B. Johnson, had a varied career in politics, government and business and was widely admired for his organizing talents. He served as Postmaster General in the Johnson Administration, and put into motion a plan to modernize the postal system, making it a quasi-private corporation. From 1975 to 1984, he was the commissioner of the National Basketball Association, and under his leadership the league made financial and rule changes that provided stability in an era of uncertainty. In 1942 Mr. O'Brien received a bachelor of law degree from Northeastern University, but he had no designs on a career in law. After Kennedy was assassinated in Dallas on Nov. 22, 1963, President Johnson persuaded Mr. O'Brien to stay on. He also served as a leading political adviser to Johnson in the 1964 election, which Johnson won with a plurality of 16 million votes. Mr. O'Brien, who lived in New York City after leaving politics, is survived by his wife, their son, Lawrence, who lives in Washington; a sister, Mary Placzek of Wilbraham, Mass., and two grandsons.