Charles O’Brien, a close associate of the union boss Jimmy Hoffa who spent decades denying that he was involved in Hoffa’s disappearance and presumed murder in 1975, died on Thursday at his home in Boca Raton, Fla. He was 86.

His stepson, Jack Goldsmith, a law professor at Harvard University, said the cause appeared to be a heart attack.

Mr. O’Brien, widely known as Chuckie, was a child when he first met Hoffa in about 1943, and the two became close. Hoffa referred to him as “my other son,” and he was Hoffa’s closest assistant in the 1950s, ’60s and early ’70s, including from 1957 to 1971, when Hoffa was president of the International Brotherhood of Teamsters.

When Hoffa disappeared in a Detroit suburb in 1975 (a judge declared him “presumed dead” in 1982, though no body has been found), Mr. O’Brien came under suspicion, with news accounts and some law enforcement authorities speculating that he drove Hoffa to a fatal encounter. Mr. O’Brien, although his accounts of the events surrounding the disappearance were sometimes vague, maintained that he had not been involved and that he would never have sold out his friend and mentor.