Having discussed this with Officer reps from BUPERS during a command visit (mid 2010, so take it for what it's worth), those "ORs" are in there specifically to set aside Nuke officer billets in STA-21. The mindset behind this is that at those points, there is a higher degree of certainty that an individual will be able to complete Power School and Prototype as an officer candidate, due to their prior success.
Most people that I dealt with that were not accepted simply didn't take the time to see the application process all the way through. It's not the easiest thing to navigate, and folks would just give up on it when they hit a snag.
Same LCDR said that this is intentional. If you can't follow the instructions provided, then this serves as an early and easy means of screening personnel. Agree or not, it would seem to hold some merit.
Your Departmental Career Counselor "should" be able to help you. Of course, if you are on a sub, there may be only 1 Career Counselor (former carrier Career Counselor here). I didn't give 2 $hit$ if you reenlisted or not because i didn't get any kind of reward or kickback for it. What I did care about was that every sailor who came to me had all the information they needed in order to make the best choice for THEM.
Best of luck, and thank you for serving!