I will recommend you to attend some classes for the exam. You will fail many times before you know exactly what to know ( and to satisfy the examiner). You will need question sets and model answers to pass the exam, as you need to recite the answer in order to finish all the question in time. I failed in the first oral exam only because I have a couple of questions left after I ran out of time. All-in-all, it is much much more difficult than Grade Two Master or a car driving licenses test (you know very much what you need to know). In compairison, I pass Grade Two Master and Engineer all in one go without going to any class, and I have classmate fail two or three times. There could be someone who failed even more than 3, if they do not totally give up.
I think you will still pass the exam without attending any class if you are persistant enough. The only problem is you probably will fail a few more times because the oral exam is really quite challenging. There is no syllabus and model questions ( not available from the marine department, as far as I am aware). This is quite different from most public exam ( like the cert exam) where may model question set is available.
I think the examiner is well aware of student attending class as they can easily tell from the way you answer the question (how can two students have the same answer in term of the exact wording if they have not attend the same school and have the same model answer?) In fact they will be a bit more demanding, and they may even try to trick you if you are one of those who do not try to digest the answer but just to recite the model answer. So be sincere and try to understand the reason behind the answer even you attend the class.