sibar,
I'm not an Officer and I've never been to OCS, but I belive appropriate civilian attire is: shirt with collar (no profanity, T-shirt is actually NOT ok. Heck, I wouldn't even wear anything with Navy/USMC on it, the DIs will have fun with you if you do), trousers with belt on (can anyone clarify if shorts with belt is ok during the summer?) and lastly, cloesed-toe shoes with socks (no sandals).
Hope that helps.
DMan