What started as an open letter from one former pop princess to a current one – in which a concerned O'Connor, 46, urged Cyrus, 20, not "to prostitute" herself – has quickly turned nasty.
In response to the letter, the "Wrecking Ball" singer took to Twitter and posted a series of old Tweets from the Irish singer, who according to BBC News, has bipolar disorder, and at the time of the Tweets was seeking recommendations from followers for a psychiatrist.
Cyrus compared O'Connor to Amanda Bynes, who is undergoing psychiatric treatment.
The response incensed O'Connor, who immediately penned another open letter, alongside other emotional responses, all of which were posted on Facebook.
"Miley … Really? Who … is advising you? Because taking me on is even more … stupid than behaving like a prostitute and calling it feminism," O'Connor wrote, continuing in another post, "Ms. Cyrus has today posted tweets of mine which are two years old and which were sent when I was ill and seeking medical help. She has done this in an attempt to deliberately cause me harm and hurt."
She continued: "Ms. Cyrus' lawyers will be contacted by mine regarding this matter. I confirm also that I do not at all support or condone the abuse or mockery of those who have been brave enough to openly discuss mental health issues. Mockery causes deaths. It is an unacceptable form of bullying."
Cyrus continued her Twitter rant, writing, "I don't have time to write you an open letter cause I'm hosting & performing on SNL this week." She then posted a picture of O'Connor's infamous SNL appearance when she tore up a photo of the pope, followed by a request to "meet up and talk."
"Miley, I have no interest whatsoever in meeting you," O'Connor wrote in a third open letter posted on Oct. 4. "You had plenty of time yesterday to abuse Amanda Bynes … an entirely innocent party … and myself … who also did nothing to deserve your abuse, along with every other sufferer of mental health problems and every person who suffered abuse at the hands of priests."
O'Connor wrote: "When you end up in the psych ward or rehab I'll be happy to visit you … and would not lower myself to mock you. Be a proper woman and make the public apologies."
Stay tuned for Cyrus's response.