The TV star bemoaned the fact that they have dropped from a DD to a B following her losing weight in recent years.
Osbourne explained that she chooses to wear Wonderbras, but that they have recently not helped her predicament.
In a series of tweets, she wrote: "Why do they call it a Wonder Bra? I think its cause when its off you 'wonder' where your boobs went! How is it I was DD at 18 & at 27 Im a B (sic)?
"Off to the gym maybe my empty sacks of s**t that I call t*t's will get even smaller! #awesome."
Kelly Osbourne recently tweeted that she was "in love" with her boyfriend Matthew Mosshart and posted a picture of them kissing.
She has also sparked engagement rumours after she was spotted wearing a ring on her engagement finger.