The fact that you were chosen for an interview is a great point in your favor. UOMHS interviews about 400 or so and admits 300, so your chances are great just starting off! Enjoy yourself, they've done most of the culling in selecting those people for an interview.
Like Laura said, enjoy yourself. They just want to get to know you. I would still prepare for the interview. After all, it's better to be too prepared then not prepared enough. I, myself, was very nervous about it until the day arrived, then I relaxed and enjoyed myself. Talk to the people, ask questions, get a feel for whether this is really where you want to go to school.
Yes, I was selected and hope to see you in August. Have fun and laugh at Dr. Murphy's and Dr. Bates' jokes. Some of them aren't too bad and it'll help you relax. C ya!
Jason