Speaking out Thursday, the Fashion Police host said the couple had come to the decision amicably.
"It was a mutual decision," she told Giuliana Rancic. "We love each other very much. It just wasn't the right time for us, and I think if we weren't as mature and adult than we are, then we could have gone down a road that wasn't really nice."
Osbourne described the split as something that "just happens in life."
"We're still really, really good friends; it's just we're not living in the same house," she said. "There's no dramatic story. I wish there was so I had something to tell people but no... it just happens in life."
After announcing their decision, Osbourne went out to celebrate her best friend's birthday and ran into none other than Justin Bieber and had good things to say about him and his spray-painting skills.
"We stop by and all of a sudden there were people spray-painting and then I got a lesson on spray-painting from Justin Bieber and he's really lovely and he's really nice, completely different than the media perception," she said. "And then I really sat back and thought, 'He ain't half as bad as I was at his age, so he's all right.'"
Osbourne and Mosshart got engaged last summer in Anguilla and planned to wed this July.