I went to OBC last year so I'll throw my thoughts out there. First of all, go to the OBC website (
http://www.cs.amedd.army.mil/obc/) and poke around for a while. The website gets updated every so often and has some good advice to prep you. Everyone says get your uniforms and boots in order before you go, and that's great advice, but it seemed like 90% of the people last year weren't able to do that. If you don't live near a base, then getting your ID and uniforms before you get to Ft. Sam could be more of a hassle than getting them with everyone else.
If you find a convenient way to get your stuff before hand, here's what I'd get:
Uniforms
2 sets of BDU's with patches (medical service corps, 2LT, your name, army, medical service shoulder sleeve insignia(USUHS patch for USUHS students), u.s. flag), 4 sets of pin on rank (2 subdued and 2 shiny), one pair of boots (I got two pair because they suggested it, but I've never used the second pair even at USUHS), 1 field cap, 1 berret (you'll probably need help shaping this), 2 sets of PT uniform (no jacket or pants needed, but the pants are very comfortable for normal wear)
Other stuff
shoe shine kit, running shoes, old pair or shoes (combatives in the mud can mess up shoes in a hurry), an all black backpack (no logos), a medium sized camelback, a little pocket notebook and pens, 20 copies of oath of office and orders, a laptop (helpful but not necessary), your car (you'll want it).
Random advice. OBC was pretty fun last year, but the first week is a bunch of hurry up and wait with very little time for sleep. You should show up in decent shape since they have a PT test that first week you show up. After that, you get into a routine where you just go to classes all day, and hang out at night/weekends. You'll probably have a lot of time to do fun stuff; someone brought their XBox, others brought golf clubs, I went to the movies. When you first get there, you'll likely try to stock up your mini-fridge with stuff, which is a good idea, but most people way over bought and had to throw stuff out at the end. The FTX (field training exercise) was dirty, tiring, and hot, but mostly fun. Part of the reason for joining the army was to do some of that stuff. We zeroed our M16's, qualified on our M9's, learned basic medic skills, and competed for the longest snot string coming out of the gas chamber. Most of your classmates will never know those joys. Some things were hard, some things didn't make sense, but it was a good experience. All in all, it was a fun time where I got to know a bunch of the people I will be working with for a good part of my life. I did hear that OBC was supposed to be getting harder, but I guess you'll see about that. About the only thing you can do now is read the OBC website, and maybe pick up a copy of the Army Officer's Guide.
p.s.
It's funny, I always seem to spend more time on this site when I have tests coming up. If I don't look at the notes, maybe the class will just go away.