For reservists, you should know exactly what MOS you will have when you enlist. Example: guy enlists active duty, gets "Aviation Maintence" occupational field. Could be anything from working on jet engines to building hydraulic lines to inspecting/repairing flight gear, but he won't find out until after boot camp. For reservists, you should know when you enlist which one of those specific jobs you will be doing since you will be filling a specific billet for whatever reserve unit you're going to.

If you have a reserve infantry company in your area you will probably end up infantry or some form of support for the company. If you have a reserve squadron close to you, you can probably get some kind of aviation maintenance MOS.

Best bet, talk to a recruiter, find out what's close to you and what they have available. Your choice, but if it were me I would give OCS another shot.