Last week, the Poker Face singer hit back at Kelly over claims Gaga's fans had sent her death threats, with Lady G slamming Osbourne's E! TV series, Fashion Police.
She said Kelly's show "breeds negativity" and encourages people to be judgemental, while also insisting that the 28-year old took "responsibility and accountability" for her actions.
Lady G wrote: "I urge you to take responsibility and accountability for the example that you are setting for young people. We can all do our part in the media to set a standard of respect, compassion, and love."
Kelly's mother Sharon then slammed Gaga's open letter, calling the 26-year old "a "[publicity-]seeking hypocrite" and "a bully", before hitting out at the Marry The Night hitmaker for making her letter public.
Shazza said: "You say her work on E! with the Fashion Police is 'rooted in criticism, judgment, and rating people's beauty against one another'. Welcome to the real world."
The former X Factor judge then quizzed Lady G over her decision to wear a meat dress and animal fur, adding: "A word from you would stop all the hideous, negative and vile threats from your "little monsters." Let me know if you want to continue this debate."
Gaga has since addressed Sharon's response on her official Facebook page, writing: "The 'real world' can be cruel, why not try to change it into a better place? I am an activist. Nobody takes adolescents seriously, I do.
"My letter to Kelly Osbourne was open, because her statements on cyber-bulling were public & as a youth activist I'm compelled to be involved."