Hi Megan,
My boyfriend just finished week 9 of OSC (yippee!) so I know exactly what you are going through. I don't know if there is a "right" answer to this, so I'll just tell you some things that I did/experienced.
First, for at least the next 4 weeks, you're not going to get to talk to him much so all of your support is going to have to be through letters. When DBF was able to call for those 15 minutes on Sunday, he would be talking so fast about everything that was going on at OCS that I couldn't get a word in!
My advice during these calls is just to listen and be supportive! Make sure he knows how proud you are of him.
There's really no point in asking many questions in your letters because he is not going to have time to write back. rhollin1 has some great OCS journals on his web page (thanks, Ron!):
http://www.geocities.com/ronhollin/ocsprep.htm
The second one is broken down day by day so back when we couldn't talk much, I would go out and read those journals to understand what DBF was going through that day (DBF's class matched the journal almost exactly). When he could call for those few minutes, I would understand what he was referring to and wouldn't have to ask as many questions.
Personality changes? He's got a potty mouth now just like a stereotypical sailor, but that's about it.
It may seem like he's a different person when you talk to him, but it's only because he is going to be so stressed out and focused. Once he's able to relax a little, he'll be more like normal.
I wouldn't worry about growing apart these next few months. His life is about to be total hell and you're going to be his "safe" spot to turn to when he does have a few minutes. It may even bring you closer together.
It's going to be tough and there are going to be days where you are going to get upset because it's going to feel like you haven't talked to him in forever. My advice for those days is to have a person you can talk to about it before you write/talk to your boyfriend. There were a couple of days when I was so overwhelmed by the situation that my letters would have just added more stress to DBF's life. Solenita's husband was 4 weeks ahead of my boyfriend in OCS (he graduated yesterday - Congratulations!!!) and understood what it was like, so she would always help me get things in back perspective.
Hope this helps some!
Kathy