Osbourne, 60, said she and Leno, 63, dated shortly after she arrived in Los Angeles from London in 1978.
"Remember, at 25, I had met my husband but we weren't together. We were just friends," Osbourne explained on Monday's edition of her chat show.
"So, I'm here, [and] I didn't have a boyfriend -- I never did date a lot at that time anyway because I was not very dateable."
Osbourne said her first impression of Leno after seeing him perform at a comedy club was he was "very nice, very funny."
She said her friend gave her Leno's phone number and she prank-called him several times, which led to him asking her for a date.
"He actually came to my house and met me and we had a little fling," Osbourne said, adding it didn't work out because Leno soon met and fell in love with his wife Mavis Nicholson, marrying her in 1980.
"The fling was more of a fling for me and not fling enough for him because a couple of months later he brought around the real love of his life for me to meet, and she was lovely and they took me to Fatburger and showed me around town," Osbourne said.
Osbourne married Ozzy, now 64, in 1982.