vwhan, by structural aspect, i mean number of faculty during the evaluation, duration, grading...I assume you've the picture now.
Quagmire, no i'm not in the UK, but I'm familiar with that system (rigorous is an understatement!). my uncle took his vive voce in Hungary waay back for his PhD in public health, but I'm only going on his info so far and i have a feeling that it is out-dated. and besides, the oral PhD exam is different everywhere. i'd just like some input on the varying experiences from individuals who have taken this path in their PhD.
As far as oral exams go, there may be 1 for the 2nd exam (program dependent) and there is one for the defense. My program gave me the option of a written 2nd exam or 1 day written, 1 day oral. I chose 2 days of written. Therefore, I can't comment much on the oral for the 2nd exam. However, I imagine the committee members will be including your advisor. So that means total 5 faculty members. I don't think other members are invited. As far as time length, I imagine it would last several hours.
The structure of the defense depends on the graduate program at that school. The format actually changed b/w years 3 and 4 for me. Initially, one would give a 20-25 minute seminar on your thesis, then your committee members (minus your advisor) and 2 readers will ask about 2 rounds of questions. Once they are done, then questions go to audience members, which may consist of other faculty, students, techs, just about any one who wants to be there. Currently, you give a 45 minute seminar. Questions go first to audience members. Once they are done with questions, they leave and 2 rounds of questions are asked by your committee members (minus your advisor) and 2 readers. The length of the defense is limited to 2 hours. Again, it all depends on the school.