I don't know of any classes, but there are study guides that give advice on improving your performance during interviews. Ask a friend, family member, or teacher to design some practice interview questions for you. Do some practice yourself. Make it realistic. Practice as you were in a real interview.

You are right to be concerned with the oral board. In my opinion, that is the most important part. They either like you or they don't. You can score high on the written test, but if you don't make a good impresison during the oral board, then forget it!