As Dwyer noted, the CNS / USCCB reviews are a good resource. And if you peruse their reviews, you will find that nudity is NOT an automatic “O”. Titanic, with Kate Winslet’s famous topless scene, got an A-III. It often depends on the context.
I recently saw the harrowing disaster film, “The Impossible”, starring Ewan McGregor and Naomi Watts. It’s based on a true story from the 2004 Southeast Asian tsunami. There’s a scene where Watts’ character, after the tsunami, is making her way through the devastation with her son, and for a brief moment her top is partially torn off, and there is partial nudity. I personally did not find this scene gratuitous, and it’s certainly not erotic (unless you get off on seeing disaster victims with their clothes falling off). It is an excellent film overall, though often hard to watch. IMHO Watts deserves the Oscar nomination she received for this film.
I could not find a CNS review for “The Impossible”, but if I was rating using their scale, I would give it an A-III - adults.