The reality TV regular, 40, spoke about the day during an appearance on comedian Jamie Kennedy's podcast Hate to Break It to Ya after his father Ozzy passed away on July 22 aged 76 after a career spanning decades in rock music and following a gruelling battle with Parkinson's disease and agonising back pain.
In the podcast interview, Jack described the final morning of Ozzy's life and said the family had not expected his death to come so quickly, and said about how the day unfolded in an ordinary way.
"It was a few days after I got back that he passed, and we were … even the morning that he passed, it was like, it wasn't anything dramatic at all," he said.
"He was up, he was doing his thing, had some breakfast, and that was it."
Despite Ozzy's long-term health issues, Jack said the family had not anticipated the timing of Ozzy's death.
"Obviously everyone knew he was sick, but it wasn't … he was really gearing up for the (Black Sabbath final) show, so he was exercising a lot and moving around," he said.
"He was fired up. But yeah, we weren't expecting it to be as quick as it was.
"I just think he was done… I do think we have a choice, to a degree."
Ozzy's official cause of death was listed as cardiac arrest, with Parkinson's disease – with which he had been diagnosed in 2003 – recorded as a contributing factor in his passing.