O'Neal was diagnosed with chronic leukaemia in 2001 and with prostate cancer in 2012.
His son Patrick confirmed the news on Instagram through an emotional post about his father.
He wrote, "Ryan was a very generous man who has always been there to help his loved ones for decade upon decade. I will share my father's legacy forever. I will not be deterred from outside voices that say negative things.
If you choose to talk shit about my dad, even though you have no clue what you are talking about, you will get called out. If you go that route, I recommend you take a good look in the mirror first."
"My dad was 82, and lived a kick ass life. I hope the first thing he brags about in Heaven is how he sparred 2 rounds with Joe Frazier in 1966, on national TV, with Muhammad Ali doing the commentary, and went toe to toe with Smokin' Joe," he mentioned in his post.
He concluded the post with, "I'll miss you dad. I love you. We love you. No one told a story better than Ryan O'Neal. 4/20/41 ~ 12/8/2023"
Later in his career, O'Neal's acting skills frequently took a back seat to media coverage of his personal struggles, which included his difficult relationship with longtime companion Farrah Fawcett, who died of cancer in 2009, and with his children, including Redmond O'Neal and actress Tatum O'Neal.
According to Variety, in the 1970s, he was a marquee draw. In 1973, he ranked behind only Clint Eastwood in terms of box office draw -- and ahead of stars like Steve McQueen, Burt Reynolds, and Robert Redford.
'Love Story' was the top film of 1970, making $106 million in the United States and Canada, and was the sixth highest-grossing film of all time at the time. The melodramatic romance, based on Erich Segal's blockbuster novel, was also a cultural sensation, with its signature line of dialogue, "Love means never having to say you're sorry," spoken both by Ali MacGraw and O'Neal at different times, inspiring parody for decades.
'Love Story' received seven Academy Award nominations, including one for O'Neal as lead actor. In 1978, O'Neal returned to co-star in the adaptation of Segal's sequel "Oliver's Story" with Candice Bergen.
O'Neal began his career in television, appearing on shows such as "The Many Loves of Dobie Gillis," "The Untouchables," "Leave It to Beaver," and "My Three Sons" before being cast as the second male lead in the modern Western series "Empire," starring Richard Egan, at the age of 22.
The actor made his feature-film debut in 1969 with "The Big Bounce," an adaptation of Elmore Leonard's novel in which he co-starred with Leigh-Taylor Young, who had been hired as Farrow's successor on "Peyton Place" and married O'Neal in 1967.
O'Neal's memoir "Both of Us: My Life With Farrah" was published in May 2012. O'Neal was married twice, both times to actresses: Joanna Moore, who died in 1997, and Leigh Taylor-Young. He is survived by daughter Tatum and son Griffin, by Moore; son Patrick, an actor and sportscaster, by Taylor-Young; and son Redmond, from his relationship with Fawcett, reported Variety.