In the accompanying story, the 26-year-old rock heiress, who admits she's never worn a bikini before, explains how being "called fat and ugly in the press almost my entire life" weighed on her so heavily that she turned to drugs and ended up in rehab.
She also reveals that her stint on "Dancing With the Stars," during which she dropped 20 pounds, made her think about health differently and propelled her to lose 30 more pounds over the past nine months. That adds up to 50 pounds lost in total.
At first, she tells Shape, hitting the gym wasn't easy.
"I would look at myself and think, 'Ugh!' I was miserable. To get to the gym when you already don't like yourself is really hard. So I had to make it fun," she says. "I started wearing cute outfits and putting on a little bit of makeup. And as vain as it sounds, it really helped me because eventually I stopped hating the way I looked."
Aside from stepping up her beauty and fashion routine at the gym, the now 5-foot-2-inch, 112-pound celeb also started taking plyometrics classes with her girlfriends.
"It's a killer -- it hurts so much," she says. "But when you walk out of there, you're like, 'I can't believe I just did that and my body looks this good.'"
And, of course, like anyone who's lost this much weight, she changed her diet as well.
"I used to eat chips and cookies and drink soda all day long," she says. "The three things I did to really change my diet were: Start eating breakfast, drink water instead of soda and always have a snack in the evening."
Osbourne reveals her daily diet, which includes a Snooki-like morning snack of dill pickles, over at Shape.com.
"I had to find things I enjoyed eating. Otherwise, I would never have stuck to the plan," she says, adding that she will allow treats every now and again. "I indulge with pizza and cheese -- I love Brie -- and have cookies sometimes. But now when I'm full, I stop eating! It may have taken me 26 years to figure it out, but I've finally learned how to do it right."
Check out more pics of Osbourne and workout tips in the December issue of Shape, on newsstands Nov. 22.
In the meantime, let us know what you think of Osbourne's bikini bod in a comment below.
Kendra Wilkinson also took the bikini route to show off her recent weight loss.
In related news: See Anna Kournikova and a host of other celebs in their bikinis.