While most of the statements in the letter were brash, O'Connor came off in good spirits. That is, until Cyrus' response sparked a heated battle.
Cyrus reacted with jabs at O'Connor's past mental illness by posting tweets comparing her to Amanda Bynes. The O'Connor lashed back in a second letter -- this time, it wasn't so friendly.
"You will yourself one day suffer such illness, that is without doubt," O'Connor wrote on her Facebook page. "The course you have set yourself upon can only end in that, trust me."
The 46-year-old singer said Cyrus' "dangerous and irresponsible" behavior sends a signal to young women that "it's OK to act like prostitutes."
O'Connor then demanded that Cyrus remove the tweets directed towards her mental state, or she will take legal action. The threats seemingly didn't affect the young singer who later responded on Twitter.
Sinead. I don't have time to write you an open letter cause Im hosting & performing on SNL this week.— Miley Ray Cyrus (@MileyCyrus) October 3, 2013
So if youd like to meet up and talk lemme know in your next letter. :)— Miley Ray Cyrus (@MileyCyrus) October 3, 2013
In a third open letter posted on Friday, O'Connor said she has no desire meeting the pop star.