You will be issued a knowledge book, which will contain all the information you'll be tested on (I'll get to academics later). You'll also be issued a candidate guide, which contains info like rank structure, billets, leadership principles, etc. If I give you stuff from the knowledge book, that's an integrity violation and no one likes an integrity violator. But the candidate guide definitely has some things that will be useful to know before hand. I'll include them in this section because you aren't testing on them, you will just use the knowledge in your day to day life.
Before OCS, I only knew the rank structure. My OSO gave me other things to learn but I didn't. Don't be like me. If you are going to learn one thing before OCS, rank structure and addressing people is definitely it. The rest can be learned while you wait in lines during check-in week, but I'll include it here because it's important.