"It's intimate, but it's going to be the sickest thing she's ever seen in her life," Osbourne tells us exclusively.
"Although I see her on the same level mentally as me, I also realize that Miley's 19 and she deserves to still have fun," the 27-year-old Fashion Police host said of her So Undercover costar. "Because that girl has so much pressure on her and has worked her ass off her entire life that I think a lot of the time people forget that she deserves to have fun too. So that was my mission for her, to make her birthday...the best day ever."
Osbourne stayed tight-lipped on specifics but did tease, "I've got all sorts of madness and craziness."
Osbourne says she's been planning the party for days and was even at the event venue until 11 p.m. on Monday working everything out.
Just last month, Cyrus helped Osbourne celebrate her birthday at the Little Door restaurant in L.A.