You actually apply for a designator before you go to OCS. That way, you know going in what you're going to be (unless you get disqualified later on). You talk to an Officer Recruiter and fill out an application, part of which is your ASTB, which your recruiter will arrange for you to take. All the applications are considered by a board of Officers who decide which ones to approve for which designators, based on quotas they need to meet, which vary constantly. Anyway, you put in your application, and they come back with either Pro Rec Y or Pro Rec N (Yes or No) for each designator you wanted. For Pilots and NFO's, the biggest things they look at are ASTB scores and GPA, so the extracurricular things usually don't make much difference. If you don't get pro-rec'd (professionally recommended) for your designator, there's no commitment involved. That's the main advantage over collegiate programs, if you don't get picked up for what you want, you won't have to redesignate, you can just end it there and either try another branch of the military or stay a civilian.