It is apparent that you don't get it. This should clear it up.
OCS, NROTC, USNA are all commissioning sources. At the completion of each program, you get a commission. For NROTC and USNA you go into it not knowing what designator (career field: NFO, pilot, nuc, etc) you will have when you are commissioned at the end. You submit a dream sheet in your senior year and get your award based on competition for open quotas between classmates from the USNA or if an NROTC guy, all NROTC grads that year.
OCS is very different. When you apply for OCS you apply specifically for a designator, like pilot or SWO. You compete on a nation wide level before routinely convened boards. You are accepted or reject for your chosen career field before you go to OCS or commit to anything. So, a guy going to OCS knows he will be a NFO, intel weenie or what ever, before he gets his hair cut. That isn't true for midshipman. You really can't compare OCS to the other programs directly when talking about "chances" of selection. An OCS applicant can reapply for his designator of choice many times until he gets what he wants or gets the hint. Midshipman get one bite at the apple. How many slots are available for NROTC compared to OCS? It varies a lot. What are your chances of getting your first choice? That varies as well, but the vast majority get what they want. The better you do in school and NROTC determines how likely you will get what you want. OCS candidates on the other hand will all be satisfied with what they have because they had a choice to accept or decline. Even if it wasn't their first choice, they took it so they had better be satisfied or they were just plain stupid. Bottom line. You can not choose between OCS and NROTC simply based on what your chances for selection a specific designator are. Clear as mud?