in my preliminary oral they tested my knowledge of my general area, and kept on until they found something i did not know too well. then they waited to see what i would do. so i chose an example, analyzed it and satisfied them.
here is on warning n what not to do:
after answering all their questions, one harmless seeming older gentleman asked out loud: are we restricted to asking only questions on this one topic? and I, unbelievably stupidly, said " ask me anything you want"
of course the next question was about a subject i knew zippo about. i manged to bluff, but do not think i fooled anyone.
needless to say, do not do that unless you wish to spoil the good impression of successful performance! just be glad to be out of there.