I don't think anyone would argue that the initial CFI checkride can be a serious pain in the arse, but just relax and look at it as one more learning experience.
My examiner didn't tell me in advance what the lessons were that I'd be teaching, but as I recall he did give me about 10 minutes to organize everything and put up some info on the board.
Good advice from Cactus about asking questions to see if they understood; don't think my examiner would have busted me for that, but who knows...
I think the big difference between the CFI oral and all others up to that point is that it's less of a situation in which the examiner asks a simple question and you give a simple answer.
For example, during my lesson on steep turns the examiner stopped me and said, "I don't understand why I get pushed down in the seat during the steep turn. Why does it feel like everything is heavier?" So, obviously I discussed load factor and drew the standard aero diagram of lift (total, vert, hor) vs opposing forces (load, wt, cent force).
Also, use whatever resources you have available to you. If he asks you a question you don't know the answer to, do what you'd do as an instructor, "Well, Bob - good question. Let's look that up - I think the AIM has a real nice picture of exactly what you're talking about..." As an instructor I do that pretty frequently; people ask really weird questions sometimes, so I just help them find the answer.
Anyway, hope this helps. Good luck!