Your examiners will not be expecting perfection and they will not be seeking to fail you.
All they will be looking for is an appropriate level of originality and research competence.
The key point to remember about vivas is that failure occurs rarely.
Assuming that the thesis was properly supervised, passing a viva is a foregone conclusion.
Unless there’s something very wrong with your thesis, you should not be and should not feel under attack. It’s meant to show that you’ve learned the essential skills of the discipline; and at the same time, have contributed something worthwhile and new (in some small way) to that discipline.
The viva, or viva voce, is an oral examination aimed at establishing that the content of a Ph D thesis is the student’s own work and that it meets a minimum standard.
Rather, rehearse answers to the questions: “What is your thesis about, what are the major contributions and what have you done that merits a Ph D? [TM:] In my case my supervisor had asked me to prepare a 20 minute ‘overview’ of my work and that’s exactly what the Chair asked me to do during the oral, but this may not be the case for everyone.
My suggestion is that you try to publish at least 3 papers in international peer-reviewed and ISI-indexed [TM: or Pubmed indexed if you are in the medical field] journals while in the process of conducting your experiments.
If absolutely necessary, it can be followed by a bit more stalling “Now the answer to that is not obvious/straightforward…” which has some of the same advantages. It is relatively common that a panel will ask one (or more) questions that, whatever the actual wording may be, are essentially an invitation to you to tell them (briefly) what is important, new and good in your thesis.
You ought not stumble at this stage, so you should rehearse this.