The Terry Richardson-directed clip features Cyrus straddling a swinging wrecking ball in the buff (save for a pair of boots) and suggestively licking a sledgehammer.
In the letter posted on her official website, O'Connor says she is reaching out to Cyrus in the "spirit of motherliness".
"I am extremely concerned for you that those around you have led you to believe, or encouraged you in your own belief, that it is in any way 'cool' to be naked and licking sledgehammers in your videos," O'Connor writes. "It is in fact the case that you will obscure your talent by allowing yourself to be pimped, whether its the music business or yourself doing the pimping."
She adds, "Nothing but harm will come in the long run, from allowing yourself to be exploited, and it is absolutely NOT in ANY way an empowerment of yourself or any other young women, for you to send across the message that you are to be valued (even by you) more for your sexual appeal than your obvious talent."
O'Connor also says she is glad she's become a "role model" to Cyrus.
"I am happy to hear I am somewhat of a role model for you and I hope that because of that you will pay close attention to what I am telling you," she writes.
"The music business doesn't give a s**t about you, or any of us. They will prostitute you for all you are worth, and cleverly make you think its what YOU wanted.. and when you end up in rehab as a result of being prostituted, 'they' will be sunning themselves on their yachts in Antigua, which they bought by selling your body and you will find yourself very alone.
"None of the men oggling you give a s**t about you either, do not be fooled. Many's the woman mistook lust for love. If they want you sexually that doesn't mean they give a f**k about you. All the more true when you unwittingly give the impression you don't give much of a f**k about yourself. And when you employ people who give the impression they don't give much of a f**k about you either. No one who cares about you could support your being pimped.. and that includes you yourself."
O'Connor continues, "You are worth more than your body or your sexual appeal. The world of showbiz doesn't see things that way, they like things to be seen the other way, whether they are magazines who want you on their cover, or whatever.. Don't be under any illusions.. ALL of them want you because they're making money off your youth and your beauty.. which they could not do except for the fact your youth makes you blind to the evils of show business. If you have an innocent heart you can't recognise those who do not."
The singer also comments on Cyrus' recent image transformation.
"As for the shedding of the Hannah Montana image," she writes, adding, "Whoever is telling you getting naked is the way to do that does absolutely NOT respect your talent, or you as a young lady. Your records are good enough for you not to need any shedding of Hannah Montana. She's waaaaaaay gone by now.. Not because you got naked but because you make great records."
O'Connor also opines on Cyrus' responsibility as a role model.
"The message you keep sending is that its somehow cool to be prostituted.. its so not cool Miley.. its dangerous," she notes. "Women are to be valued for so much more than their sexuality. we aren't merely objects of desire. I would be encouraging you to send healthier messages to your peers.. that they and you are worth more than what is currently going on in your career. Kindly fire any motherf***** who hasn't expressed alarm, because they don't care about you."
You can read O'Connor's full letter here, or here.
Update 2: O'Connor has written a second open letter to Cyrus, and threatened legal action against the singer.
O'Connor responded to Cyrus' earlier tweets on her Facebook page.
"Miley... Really? Who the f**k is advising you?" O'Connor writes. "Because taking me on is even more f*****' stupid than behaving like a prostitute and calling it feminism. You have posted today tweets of mine which are two years old, which were posted by me when I was unwell and seeking help so as to make them look like they are recent. In doing so you mock myself and Amanda Bynes for having suffered with mental health issues and for having sought help."
"I mean really really... who advises you?" O'Connor continues. "[H]ave you any idea how stupid and dangerous it is to mock people for suffering illness? You will yourself one day suffer such illness, that is without doubt. The course you have set yourself upon can only end in that, trust me. I am staggered that any 20 yr old woman of the 21st century could behave in such a dangerous and irresponsible manner as to not only send the signal to young women that its ok to act like prostitutes but also to the signal that those who have suffered or do suffer mental health problems are to be mocked and have their opinions invalidated. Have you no sense of danger at all? or responsibility? Remove your tweets immediately or you will hear from my lawyers. I am certain you will be hearing from all manner of mental health advocacy groups also. It is not acceptable to mock any person for having suffered."
In another post, she adds: "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. Period. It is an unacceptable form of bullying, no matter who it is doing the bullying."
Cyrus has also taken to Twitter to respond.
"Sinead. I don't have time to write you an open letter cause Im hosting & performing on SNL this week," she wrote.
"So if youd like to meet up and talk lemme know in your next letter. :)"
Update: Cyrus has responded to O'Connor's letter by comparing the singer to troubled actress Amanda Bynes, who is struggling with her mental health.
She later shared an image on Twitter of O'Connor tearing up a picture of Pope John Paul II during her infamous Saturday Night Live appearance in 1992.
Meanwhile, Cyrus has said that she is ready to "let go" of her squeaky-clean Disney image.
"I just needed to let go of the past in a way," the pop singer reveals in Miley: The Movement, a new MTV documentary that aired in the US this week. "I just needed to let go of the past in a way," she says, "so I felt like I could be the bad bitch I am."
Check out the trailer for the doco below:
NOTABLE NUGGETS
And finally ...
* Frank Sinatra's widow on Mia Farrow's paternity bombshell: "It's just a bunch of junk".
* If anyone ever tells Jennifer Lawrence to go on a diet again, she'll tell them exactly what they can do.
* Paris Hilton is taking another stab at her music career.
* David Brent gigs sold out as Ricky Gervais slams "f***ing disgusting" ticket touts.
* Lady Gaga went on "secret" date with Johnny Depp? I should coco.
* What do Benedict Cumberbatch and Emma Watson have in common?
* "Immature" Justin Bieber won't be charged for allegedly spitting at his neighbour, reckless driving.
* Daniel Radcliffe talks separating himself from Harry Potter.
* Joseph Gordon-Levitt doesn't think he'll be in Batman Vs Superman movie.
* Watch: Usher remixes the ABCs on Sesame Street.
And a glorious weekend to you all!
- Rumour Has It
