Awhile back, I posted some advice from my own experiences on doing a PhD.  Since then, several people have asked me for advice about the viva voce (or oral) examination, which most PhD programs require at the end of the degree.    Here are some notes I wrote for a candidate recently.
It is helpful to think about the goals of the examiners.   In my opinion, they are trying to achieve the following goals:
1. First, they simply want to understand what your dissertation says.   This means they will usually ask you to clarify or explain things which are not clear to them.
2.  Then, they want to understand the context of the work.  This refers to the previous academic literature on the subject or on related subjects, so they will generally ask about that literature.  They may consider some topic to be related to your work which you did not cover; in that case, you would normally be asked to add some text on that topic.
3.  They want to assess if the work makes a contribution to the related literature.    So they will ask what is new or original in your dissertation, and why it is different from the past work of others.  They will also want to be able to separate what is original from what came before (which is sometimes hard to do in some dissertations, due to the writing style of the candidate or the structure of the document).   To the extent that Computer Science is an engineering discipline, and thus involves design, originality is usually not a problem:  few other people will be working in the same area as you, and none of them would have made precisely the same sequences of design choices in the same order for the same reasons as you did.
4.  They will usually want to assess if the new parts in the dissertation are significant or important.  They will ask you about the strengths and weaknesses of your research, relative to the past work of others.   They will usually ask about potential future work, the new questions that arise from your work, or the research that your work or your techniques make possible.  Research or research techniques which open up new research vistas or new application domains are usually looked upon favourably.
5.  Goals #3 and #4 will help the examiners decide if the written dissertation is worth receiving a PhD award, since most university regulations require PhD dissertations to present an original and significant contribution to knowledge.
6.  The examiners will also want to assess if YOU yourself wrote the document.  They will therefore ask you about the document, what your definitions are, where things are, why you have done certain things and not others, why you have made certain design choices and not others,  etc.     Some examiners will even give the impression that they have not read your dissertation, precisely to find out if you have!
7.  Every dissertation makes some claims (your “theses”).  The examiners will generally approach these claims with great scepticism, questioning and challenging you, contesting your responses and arguments, and generally trying to argue you down.   They want to see if you can argue in favour of your claims, to see if you are able to justify and support your claims, and how you handle criticism.   After all, if you can’t support your claims, no one else will, since you are the one proposing them.
The viva is not a test of memory, so you can take a copy of your thesis with you and refer to it as you wish.  Likewise, you can take any notes you want.    The viva is also not a test of speed-thinking, so you can take your time to answer questions or to respond to comments.    You can ask the examiners to explain any question or any comment which you don’t understand.   It is OK to argue with the examiners (in some sense, it is expected), but not to get personal in argument or to lose your temper.
The viva is one of the few occasions in a research career when you can have an extended discussion about your research with people interested in the topic who have actually read your work.   Look forward to it, and enjoy it!