I had my oral exam already so this thread isn't relevant to me anymore, but it may be of use for someone else.
My oral exam was definitely more of an interview and centered around what sort of research I plan to conduct at the university. There was a board of professors (maybe around 10?) who asked me various questions about my research plan. Some questions were particularly inquisitive and difficult to answer, especially those regarding viability. But the atmosphere was not hostile. Also, my Japanese ability was not a concern to the board.
Fortunately I came prepared with a formally written research proposal. In the end, my oral exam was a success and I have been accepted into the program. For those applying to a research based graduate program in Japan, I suggest writing a research proposal following formal formatting guidelines. It wasn't required for entry into my program, but the oral exam board was certainly impressed. Furthermore, I had been in close contact with my prospective supervisor. I highly recommend that as well.
Of course, keep in mind that all programs are different and so my advice may not apply to all those about to take an oral exam. Nonetheless, this has been my experience.