The questions are usually pretty similar from an agency to the next, if certified expect a few questions about yourself, your background, what you know about the agency and jurisdiction, why do you want to work here and such things, and probably some job related questions, is not certified an applying for sponsored position probably the above except for the job related questions. I don't think there is much preparation to do for an oral board, if you've been working as a LEO you should be able to answer questions about the profession, and for the rest either you present well, you communicate well, you appear honest, motivated etc, or you don't and I doubt you can achieve that in days/weeks just by reading some forum.
Good luck to all