He was 82.

The cause of his death is still not known.

The actor was reportedly diagnosed with chronic leukaemia in 2001.

He was also diagnosed with prostate cancer in 2012.

Confirming his death, actor's son Patrick O'Neal wrote on Instagram: "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."

"Ryan never bragged. But he has bragging rights in Heaven. Especially when it comes to Farrah. Everyone had the poster, he had the real McCoy. And now they meet again. Farrah and Ryan. He has missed her terribly. What an embrace that must be. Together again," he said.

In 1964, he landed the role of Rodney Harrington on the ABC nighttime soap opera Peyton Place.

It was an instant hit and boosted O'Neal's career.

He later found success in films, most notably in the romantic drama Love Story (1970), for which he was nominated for the Academy Award for Best Actor and the Golden Globe Award for Best Actor – Motion Picture Drama,Peter Bogdanovich's What's Up, Doc? (1972) and Paper Moon (1973), which earned him a nomination for the Golden Globe Award for Best Actor – Motion Picture Musical or Comedy, Stanley Kubrick's Barry Lyndon (1975), in which he portrayed the titular character, Richard Attenborough's A Bridge Too Far (1977), and Walter Hill's The Driver (1978).

From 2005 to 2017, he had a recurring role in the Fox television series Bones as Max, the father of the show's protagonist.