Osbourne was criticizing Aguilera's performance look at last weekend's Michael Jackson Tribute Concert. The 30-year-old paired a pantsless ensemble with fishnet tights, heavy makeup and supersized hair.
But Ozzy's daughter wasn't the only cohost to attack Aguilera's figure. "Lady Marmalade got into the peanut butter again," Joan Rivers sniped.
"I'm looking at this and thinking, 'Well, she's put on weight. This just isn't flattering,'" George Kotsiopoulos said. "But this is just a crappy outfit, regardless. No matter what shape your body is in, it's just bad."
Kotsiopoulos pointed out Aguilera was "still probably a size 2/4," but Osbourne was quick to disagree.
"Trust me," she said. "I'm a 2/4. That is not a 2/4."
This isn't the first time Osbourne has criticized The Voice coach. Last month on E!'s Fashion Police she called Aguilera a "fat bitch."
"She called me fat for so many f***ing years," Osbourne vented, "so you know what? F**k you! You're fat too."
Ironically, Osbourne battled with her weight in the past, but after her 2009 stint on Dancing with the Stars she shed 48 pounds and kept it off. "I took more hell for being fat than I did for being an absolute raging drug addict. I will never understand that," she told Us in 2010. Osbourne checked into rehab for drug and alcohol addiction three times.