He told Britain's BBC Radio 1 Legends show: "The world stopped for me when Sharon told me that John Lennon had been shot in New York. I can't even describe how I felt.
"If he was a politician, a soldier, anything else, it would be, 'Oh yeah, well he is in that kind of business,' but the amount of joy and hope that he gave people was just remarkable."
Ozzy, 59, also said he would have loved to see the Beatles perform, and thinks they would have reunited if all four members - John Lennon, Ringo Starr, Sir Paul McCartney and the late George Harrison - were still alive.
He added: "I often daydream about whether they would ever have reformed. I never saw the Beatles, I saw Paul McCartney play but never saw the Beatles - I would have loved to have seen them play."
Lennon - an ardent peace activist - was shot dead outside his New York home on December 8 1980 by obsessed fan Mark David Chapman.
Harrison died in 2001, aged 58, after battling lung cancer and a brain tumour.