How to prepare for your PhD viva

Anyone who has never made a mistake has never tried anything new -- Albert Einstein (1879-1955)

1. Mental preparation: Fill yourself with confidence. You have prepared the thesis for so many years (since you started your research), but the examiner has only studied it for a short period of time. You are a world expert in the thesis (otherwise you don't deserve to pass, and I wouldn't have supported your submission). You are there to tell the examiners something substantial and significant.
2. The examiners' job: The examiners' primary jobs are to establish: (a) whether the thesis has made substantial and significant contributions to the subject; and (b) whether the thesis is your work, not someone else's. Question (a) is mainly answered by the thesis, but the examiners may need to seek clarification during the viva. Question (b) is mainly answered through the viva; for this reason, you must know your thesis inside-out.
3. Format of the viva: The examiners will normally ask you to summarize what you have done in the thesis. This is your chance to set the agenda: you want to focus them on what you think are important in the thesis. Questions often follow your summary. The best preparation is to write down your summary (if you haven't done so yet), practice reading it out until fluent.
4. Errors: Don't worry about errors in the thesis at this stage. There are always errors. Of course they give examiners justifications for saying "the thesis is not ready yet", should they lean towards failing the thesis. But don't let this worry you and dent your confidence.
5. Serious Omission/Errors: If you find serious omissions (or errors) in your thesis, the best way to handle the situation is to write a new section (revised version in the case of errors) and bring it to the viva. If asked, you can show that you are aware of the error, and you will correct it. This new section will eventually be added to the final version of the thesis (or replacing the relevant sections).
6. Assessment: The examiners will assess your ability to defend your thesis. Don't be alarmed if they ask tough questions. Sometimes they don't have answers to the questions themselves. Examiners often use such questions to assess the depth of your knowledge.
7. Answering questions: Listen to the examiners' questions carefully before answering. It is a good idea to pause for one second after a question is asked. Don't hesitate to ask the examiners to repeat their questions. You may also rephrase the questions to seek confirmation from the examiners. Answering the wrong question could make the examiners believe that you don't understand the subject.
8. Scope of the research: The examiners (due to their own interests) may ask you questions on issues which are not central to your thesis. When this is the case, explain that this is out of the scope of your thesis.
9. When to defend and when to concede: The examiner may criticize part of your work. When the criticism is on issues fundamental to your major claims, you must defend yourself clearly. You may concede on issues that are not central to your claims. Conceding everything or defending the undefendable are both serious errors.
10. Bibliography: Nobody expects you to remember every piece of work that you have cited. Obviously the more you remember the better. You must know the core references inside-out, but don't lose confident just because you can't remember minor references.

Related:

• Commonly asked questions in a PhD viva