One thing I noticed is that if you have written about somthing in your primary or secondary that is generally one of thier interview questions, they won't re-ask. For alot of schools it seems like they sometimes don't read your secondary based on their questions, but I had written a alot about DO / OMM and didn't have any of those questions in my interview. It may seem like common sense, but some of the questions that I was really prepared to speak about weren't asked.
Overall though the interview was laid back. You should have nothing to worry about.