Later on Instagram, the TV presenter took a shot of her "Robocop boot" and revealed she had suffered a stress fracture.
"#StressFractures are the worst! At least I get a cool #RoboCopBoot", Kelly captioned the snap.
The 29-year-old star was attending the 2014 Trevor Project NextGen Prom with pal Amanda Golombek and had styled her recently shorn off hair in to a stylish quiff.
Wearing a gorgeous black lace dress, Kelly showed off her svelte figure - which she maintains through healthy eating and regular workouts with a personal trainer.
After Kelly split from her fiancé Matthew Mosshart she was said to have gained weight. But she disputed this - and shared her annoyance over the media's obsession with her figure.
Kelly Osbourne gave fans a closer look at her foot brace
"When did I all of a sudden gain 20lbs?" she tweeted. "I just asked my ass and she can't seem to remember either! #MustBeASlowNewsWeek!"
The Fashion Police host took to Instagram to update her fans as her hair underwent another transformation.
While she had previously only shaved sections of her head, Kelly decided to fully shave both sides, leaving a mohawk of purple hair on the top.