I know the Oral prep is expensive but I think the Oral prep can install a lot of knowledge, and you should also read the books above as well to give you the best chance of passing. Make sure you study Radar and the W.O.A triangle sufficiently and rule 19 (you will be tested on these) here are a couple of good links I found before my Exam, one you can print off Radar plotting sheets and practice basic plots as well as more advanced ones
SeamanshipTutor.Com the other website gives you some examples of questions asked in MCA Orals
MCA Oral Exams sent to me by Candidates
PM me with your Email if you want me to send you a couple of more things that may help you.
Good luck!