As I did (just talking about my own personal experience, don't wan't to generalise). The day before I went to my OD: got myself an appropriate haircut. Make sure you're shaved too.
The OD itself I went in (black suit + brown belt + black shoes). Make sure you have a VERY GOOD looking close up picture in business attire
: with nice (white teeth), smile, shaved!
I also put a full length photo (business attire) included with my resume. One more tip: try to profile yourself as being different without being dominant or act like a 'boss'. Be laid back, don't stress out in other words, be yourself: be social! Not too much not too many. It was my first job interview in my life and I succeeded so... . Good luck!