The Rocket Man has planned a series of lavish get-togethers in Los Angeles for Furnish's half-century milestone, inviting superstar guests from around the world and booking Lady Gaga, Cher and the Scissor Sisters to perform at various events.
But Osbourne, a close friend of Sir Elton's, has turned down her invite as she wants to spend her 28th birthday on Saturday (27Oct12) eating "yummy English food" with her pals.
She tells U.K. talk show Daybreak, "I was invited to all of the festivities but unfortunately every year I have to pick between going to David's birthday or having my own birthday. So I decided that I'm actually going to be coming home, but I can't wait. He's going to be mad at me but I wanna see my friends!
"I cannot believe that I'm turning 28 this Saturday, it's insane. I look back at my life and I'm just like, I still feel like I'm 12! And I still act like I'm 12! I'm like a little upset that I'm not going to be there but I'm going to be with my friends (and) I can't wait to have some yummy English food."
David and Victoria Beckham and British actress Elizabeth Hurley are among the stars rumoured to be celebrating with Furnish, who turns 50 on Thursday (25Oct12).