Always been a fan of your posts, Sir.
My experience at OCS was a great example of the OP's worry. I had a full flight physical with the Marine Corp @ Miramar prior to attending OCS, so I knew I was within limits (20/20, good depth perception, everything fine). Come OCS, it was the second/third? day in, we were all SUPER-hydrated, exhausted and just trying to stay awake. For many, falling asleep in the eye doctor's chair was an accident and not intentional. I was not an exception. Turns out that people's bodies react differently to this (I compared my experience to shipmates later) and I experienced a somewhat impressive reduction in my vision. Mind you, I could still see and fortunately I was within limits, but the tests revealed that I was 20/40 at the time and right on the limit for astigmatism without a waiver (whereas normally I am well within limits). Not only that, but my EKG read abnormal which would have put me in medical limbo for most of OCS (assuming they had done more tests and found nothing) if I hadn't had that normal EKG when I was at Miramar. The bottomline of all this is that OCS medical determined that I would have to wear glasses to fly. My attitude at the time was "so what, I still fly, no big deal." When I came down to Pensacola for my short form, I did the vision tests again and everything was back to normal. The different results between tests was enough to get the attention of one of the lead docs who spoke to me briefly and after being assured that everything was normal and I really didn't need glasses, he let me go on my way.
As I said, not everyone will have the same experience, but I can definitely see other people falling outside of limits. Supposedly, they can retest you later in OCS (during candio) when you are not as exhausted, but I honestly couldn't tell you if that was any better.
My 2 cents.
edit: hydration
