I wouldn't be too nervous - although I was at the time. Made a big faux pas though by removing my suit jacket and hanging it on the back of the chair before the interview. The interviewer wasn't impressed!
About interviews - the more you do, the better you get. Like anything, experience counts for a lot. Between filter interview and OASC, I'd put yourself a plan of action together including getting as much interview experience as possible. You might find friends/family can help you out with this. For example, I had a contact at a
PR company who put me through a few interviews and gave me good debreifs on what they thought. Its worth going out of your way for!