I just interviewed for a PO1 job in Southwest MO and just as the previous post mentioned, they asked questions looking for a basic, general knowledge of human and criminal behavior. Your extensive education in Psych will help you with that (my degree is in psych and I was told after the interview that I did the best interview of the round). The last part of the interview is situational; they will pose a situation and ask you how you would respond. You will be interviewed by a panel made up of three supervisors. If the interview is successful, you will be called for a background interview within two-three weeks. After that, they will notify you of the outcome within two weeks.
I too like to be prepared so I spent several days looking up potential interview questions, only to find out during the interview that the questions for which I prepared weren't even mentioned! Your best preparation is the ability to communicate extremely well and with confidence (but not arrogance), enough self-awareness to understand why you want to be a PO, a good night's sleep and some deep breathing before your interview. And get rid of the idea that you need to answer each question "correctly" or perfectly. Trust that you already know the answer and you will be fine! You are obviously intelligent; keep reminding yourself of that on the way to the interview.
My only question for you is why, with your ambitious and admirable educational goals, would you want to work as a PO? In certain areas of MO, most PO's work with probationers, not parolees. I imagine you would get so much more experience working and studying the criminal mind by working corrections in a prison setting rather than in a community setting. Keep in mind that you don't need to work as a PO to get some PO training. Most Missouri PO offices allow volunteers, where you will work with a current PO and serve him/her as an assistant. This is also an option if you, for some reason, don't get offered a position. Volunteering for 150 hours will give you an extra 5 points on your Merit score, which will improve your future chances at a position.
Hope this helps. Good Luck!