I had an interview at ICO a few months ago, and I was really nervous about mine too! But really it is sooo not as stressful as it seems. Most of the interview day you're with a group of people, the only parts that are one on one are the file review and the actual interview. For the file review you just go over your application and make sure everything is in. Even the actual interview is more laid back than I thought it would be. It really was like a conversation, and although I heard that it varies depending on which interviewer you get, I haven't really heard any horror stories.
The only advice I would give you is to be as prepared as you can be by reading up on the school and thinking of what you would say to possible interview questions. Then just relax and try to be yourself. Everyone at ICO is very friendly and will try to put you at ease.
Good Luck!