As the others have said, you can transition from one platform to another. I transitioned from EP-3E's to EA-6B's 3 years ago. You incur a five year commitment at the end of your training, the FRS (they changed it from 2 1/2 years right befroe I did it). You incur 2 years if you fail, and some people do unfortunately. It is not an easy or hard process.
You need 3 things to transition
1-A letter stating why you want to transition, what your primary and secondary choices are and your qualifications are.
2- To be physically qualified (some people are qualified only for P-3's/Helo's).
3- A letter from you CO recommending you for the transition. This is the hardest to get. A lot of CO's take a very dim view to someone who wants to leave their community. My CO was great and supported me all the way, I have known some that have shot down applications.
You then go to a board that looks at your application and decides whether or not you transition. I was told by my XO, a transition from helos to EP-3's, that it is more like the 2 detailers get together and make a recommendation to the board that is almost always taken. They see if your year group in your original community has enough people and if the one you are going to has enough room. After that you are sent to training, which differs on which platform you go to. I was sent VT-86, the advanced training squadron for NFO's and then on to the Prowler FRS.
The Navy usually has 2 transtion boards a year and they select around 10-20 people for each board, both Pilot and NFO. Usually 80-90% pilot, not too many NFO's. The last board was in Jan 2002, they have cancelled the last few bacause of the S-3 sundown.
There is a limited time in which you can transition. You have to be fuly qualified in the aircraft you are transitoning from, which takes up to two years from the completion of the FRS. That is about your 5 year point. You have to do it in time so that you can complete a junior officer tour in your new community, about 2 years. You would have to get selected by the end of your shore tour, about the 8 year point. I have known several guys who have transtioned later but they usually had a connection or two, one was an admirals aide (damn good pilot though).
Some communities are nicer to transtions than others. There are a lot of former helo pilots in Prowlers because they still have a pretty good chance at command. The XO (Executive Officer/second in charge) in my new squadron is a former helo bubba. Some communities are not as nice to transitions, if you are not born and bred then you are not a true xxx pilot.
As for OCONUS sites for the Navy, there is an Air Wing in Atsugi, Japan near Tokyo. It is attached to the USS Kitty Hawk that is in Japan. Ther eis no Naval Aviation in Korea though. Of course, you will OCONUS in any platform but the E-6, it is called deployment.
As for the number of Helo pilots in the Navy, I was suprised to learn that they are only 25% of the pilots in the Navy. Our XO (the former helo guy) worked for the Bureau of Personel and gave us a little slide show on aviation in the Navy and had a pie chart on the makeup of aviators in the Navy. It was something like 25% P-3, 25% Jets, 25% Helo, and the last 25% the rest (for pilot).
The last thing I wanted to say about transitons is that a lot of people who do not get what they want in flight school say they will transition at first. A lot of those guys lose interest after a few years for several reasons though. The main one being they become established in their community and don't to start all over again. I went from being a Mission Commander to a lowly flight student in 2 weeks, the only one with wings in VT-86. It was a little humbling. I know of a lot of guys personally lost interest but a few others who never did. For me, I have not regretted it for a moment. Flying a jet is 10 times better then sitting sideways in a tube staring at a computer screen. Best of luck.
