Just to clarify, you are designated, in the eyes of the medical standards, once you walk out with your upchit. This happened for me AT OCS (because I got my full flight physical on day 2 of OCS). After I commissioned and went to branch for my upchit, I simply met with a flight doc. He asked if anything had changed, discussed a medical waiver I had, and handed me a new upchit. I asked him at THAT point what my standards were. He told me that I was officially designated SNA AFTER he handed me the upchit and that it was now 20/100. I'm sure it's buried in some document somewhere, but this was straight from the horse's mouth (flight doc).
For OCS guys NOW? I have no idea. I'm sure they have a system for flight physicals up north and it's similar now with just a formality to meet with the doc to get an upchit.
I DID NOT require another "physical" after OCS. I just met with the doc and got an upchit.
This is also similar to getting assigned to a new squadron/wing/whatever. When I came to Corpus, we all just got our new vaccinations, fixed anything that needed fixing with our records or got anything we needed done and then got a new upchit handed to us. No formal physical, just formalities taken care of and a fresh upchit delivered, even though we had ones from our previous command. (This happens when you switch commands. Like OCS to API was OTCP to NASC and API to Primary in Corpus was NASC to CTW4) Hopefully this sheds some more light on the process.