Probably the most unnerving thing about an oral examination is to be asked a question to which you do not know the answer. Your first reaction will be to fake it, to come up with something that will disguise your ignorance and make you sound sufficiently brilliant. Ignore your first reaction. Your second reaction will probably be to deflect the question in some way, to divert the questioner onto some other issue about which you are better informed. Ignore your second reaction. If you really don’t know the answer to the question, you must say so. As I said in my last post, trying to fake your way through an answer actually looks worse than saying “I don’t know.” At least an answer like that demonstrates that you understood the question and are aware of the limitations of your own knowledge.
The key to saying “I don’t know” is to recognize that this is actually a part of the process. I’m not sure about other examiners, but it is actually my goal in an oral examination to get you to the point where you have to say “I don’t know.” And, I don’t do that simply to torture you. (The torture is just a perk of the job.) We all have limits to our knowledge, and one of the goals of the examination is to find out where your limits are. So, “I don’t know” is actually an expected part of the examination. It’s not a question of whether you’ll say “I don’t know,” it’s a question of when.
I should also admit that we are not beyond asking questions for which there is no good answer. (I am particularly prone to throwing in questions like this just to see what you’ll do with them.) If you get a question like that, feel free to say that you don’t know the answer because no one really does. If you want to go on from there and offer your speculative attempt to answer the question, go ahead. Just make sure that you clearly indicate that this is just your attempt to answer the unanswerable.
Having said all of that, of course, you want to avoid saying “I don’t know” too quickly. If you’ve prepared decently for the exam, you will be able to answer most of the questions adequately. So, if you think you don’t know the answer the question, make sure that you understood the question. There’s nothing wrong with asking the questioner to repeat or even rephrase the question. A lot of the questions that get asked in an oral examination are spontaneous, so they’re not always carefully crafted. It’s entirely possible that the question is simply unclear. Make sure you understood it properly before admitting that you don’t know the answer.