Getting into USN
Based on your location, I would have to note that first off you have to be a US citizen to become an officer in the USN. To apply through OCS you have to complete a college degree (BS/BA). Technically there are no differences between the different commissioning programs, since everybody is now commissioned a reserve officer and you have to apply to convert over to regular.
During the application process for OCS, you can tell the officer recruiter you want flight school. They have some extra tests to ensure that you qualify, plus the medical exam. So the answer is yes, you would know before going to OCS that flight school would follow. In that regard, ROTC isn’t as nice because you have incurred a commitment before service selection. Also, once you have orders to flight school you receive another flight physical in Pensacola at the Naval Aviation Medical Institute, who’s mission seems to be “We provide Naval Flight Officers to the Fleet”.
If you don’t make it through flight school then you have a problem. You signed up for a commitment to Uncle when you went to OCS. Not sure what they are doing now, but Ensigns who flunked out of flight school were being sent on two year orders to carriers to make up for the lack of LT/LCDR available at that time (2000-2002).