"I have never felt this close to another human being, ever. We're completely in love," the 28-year-old "Fashion Police" co-host told Hello! magazine. "People say we're co-dependent, but we just want to be together all the time."
Osbourne's brother Jack told People.com in February she was engaged and intended to marry Mosshart, 29, in the summer of 2014, but she had not confirmed her betrothal until now.
"We're thinking, 'Let's just have my mum plan it and we'll just show up.' She's the best at that kind of stuff," Kelly Osourne told Hello!, referring to her mother, Sharon, who is a co-host of "The Talk."