Depending on your committee, you want to try to talk with your readers rather than at them. The most positive exam experiences are those in which the oral turns into an academic discussion rather than a grilling session. If you can talk personally and personably with your committee, there is a better chance of this happening. So it is important to relax, ask questions, and be yourself while at the same time keeping in mind that you are being evaluated on your ability to field questions with confidence and maturity.
Don’t bluff. Some oral exams, unfortunately, do become grilling sessions anyway. If you are asked a question to which you don’t have an answer, don’t pretend that you do. You will open yourself up to intellectual attack and the examiners will wonder about your academic honesty.
Don’t be afraid to say you don’t know. Chances are you will be thrown some questions you can’t answer. You can reply that you don’t know, but try to direct the questioning elsewhere with your reply. Saying "I don’t know about that particular author, but I know that this author sets things up in this way," or "I’m not sure about how that might be applied in this particular situation, but I know another example where that might be the case." In other words, do not be afraid to direct the conversation to what you do know. Act with confidence. But also know your limits. If the examiners are hell-bent on pursuing one line of thought, let them do it and just do your best. Stay calm and honest. Also, ask for clarification of the question; the examiner might elucidate in such a way that triggers something in your memory or helps you understand the question in a light that enables you to answer it after all.
Before you go into the exam, try to imagine what the larger questions might be. While some examiners may want to get into the details of what you’ve written, you should also be prepared for meta-questions, such as:
Where do you position yourself as a scholar in this discipline?
Do you see any intersections among these lists/scholars/texts?
What are the important works/themes/scholars which have defined your reading?
Where does [insert seminal text here] fit within the more general evolution of the discipline?
Are there still gaps in your knowledge that you would like to research further?
Where do you see your work going in the future [i.e., have you thought about your dissertation since that’s the next step in the process]?
Align yourself as a scholar. While a lot of exams are surveys intended to test the breadth of your knowledge, you will undoubtedly be responsible for a certain depth of knowledge as well. Work to understand the strains of theoretical thought in your discipline, the large debates, the different methodologies and paradigm shifts. Then think about how you align yourself as a scholar. This is particularly important if you belong to a department that requires an oral exam. You may be asked about such debates or ruptures in the history of your discipline and then more importantly asked with which side of those debates or with which methodological/theoretical approach you align yourself. It is a good to have this answer prepared; even if you are not asked such a question, you have a better sense of yourself as a scholar as you go into writing the dissertation, the next step on your scholarly journey toward the PhD.