Few clarifications...
Yes you take the ASTB BEFORE signing your contract, however many times it takes to pass...
Air contracts do not have a seperate OCS, they just attend with everyone else. So for a timeline this is what it will look like... ASTB -> Sign contract -> OCS -> Sign new / commissioning contract -> Get commissioned -> TBS -> Pensacola for Flight School.
Yes, selection for OCS once you have completed college (ie OCC) will be much harder and you will just have to be that much better or expect to have to apply more times and therefore wait longer. OCC is a 10 week course that can occur throughout the year.
The PLC option has two methods of going through OCS but since you are a Junior you are limited to just one of those. The only option you can do at this late point in your college career is to go for a single 10 week course during the summer, typically Charlie Company June - August. Now if you had applied and been accepted earlier then you would have been given the option / been forced to go to 2 seperate 6 week courses.
Not sure if you asked but there are two contract durations, both of which START AFTER you get your WINGS (ie 2+ years into your Marine Corps career). They are dependant upon which airframe you select (helos = 6-8 yrs, Jets = 10 yrs give or take...).
Hope this helps, if not feel free to ask any further questions...
