Probably the most important thing that made it a good process was that, although I was certainly under pressure to perform and show that I knew and could explain the material, the format was as a thoughtful, two-way, deeply-engaging conversation rather than a grilling, one-sided examination.

The examiner (one of my two instructors) had a list of questions/topics that we had to get through, but the specifics and flow were natural and spontaneous rather than artificial and forced.

What made the conversation good was that the examiner discussed points that I raised, raised points that I didn't, asked my opinion on e.g. real world implications of theories or conclusions that could be drawn if this or that were true, et cetera. This made it two-way and engaging. While he did not give me any answers, of course, when it was clear that I could bring up and sufficiently discuss a topic, then he would go into details, which would trigger even more detailed responses from me, and so forth. In this way I think he was able to probe the depths of my understanding while not needing to employ a one-sided question/answer format.

I think that it is difficult to bullshit a topic in depth with someone else who knows what they are talking about, so an oral exam probably does not need to be a hardcore opposition like a thesis defense might be.

The allotted time was maybe 30 or 45 minutes (I don't quite recall), after which the examiner would tell you what grade they would recommend. If it was a grade with distinction, then of course you would say thanks and be done. If it was less than distinction then you could request another 10 or 15 minutes of further examination to try to show your mastery. (I passed with distinction, so I didn't go through that part, but I assume it would have been a continuation? But maybe it would get a little more intense if you were trying to improve your performance at the last minute? I don't know.)