It's been a while since I did OBC, but unless it's changed dramatically, you will:
1. Sit around in classes all day listening to lectures that for the most part have nothing to do with medicine (my favorite was the class on Soviet artillery).
2. Stand around in formation every so often. Learn how to march, left face, about face, etc (takes about an hour).
3. PT a couple times a week at 0500. Jumping jacks, pushups, situps, 2-3 mile run that's broken into groups based on ability.
4. Spend a week "roughing it" at Camp Bullis (live in huts or tents, run around with field gear, etc).
5. Get to know your fellow OBC people. You'll see lots of them again in residency, future assignments, etc.
6. Have fun in the San Antonio area after hours and on weekends (it helps to have a car).
Basically, don't sweat it.