OR is definetly something you could do. They're constantly trying to get RNs to sign up for OR training. Seems there's always a shortage, at least on the Navy side. PACU and cath lab would be harder simply because the PACU and cath lab are desirable places to work. They won't even consider you for either unless you have ICU experience. I'm sure there are exceptions but that's what I've seen.
I work at one of the big 3 hospitals in the Navy. If you have prior experience in any of these fields then your chances of working there are much more likely. To work in the PACU at our facility as a RN you DON'T have to have ICU experience. In fact I think that they only have civilians in our PACU. They have many enlisted folks there but no AD nurses. They moved all the AD nurses to other areas of the hospital. As for the OR, I don't know much about that. Just remember that nothing is for sure in the military, despite what they tell you.