My experience is that the lower your test score, the more questions you may get in the oral portion.
The biggest thing I learned, especially when I got my IFR rating, was don't BS any answer. If they hit on a topic you don't know. Tell them that you are not totally sure of the answer, but you know where to get the answer. Look it up in the AIM, FAR, POH, etc that you have with you in front of them. IF you show you know the manuals well enough to look something up quickly, it will usually not be held against you. Just don't do it for every question. Most reasonable FAA examiners understand that you won't have the entire AIM or FARs memorized. WIth that said, there is also that one exception. Hopefully that isn't the examiner that you get.
Also, ask others who have taken rides with your examiner. If you learn they are a jerk, schedule with another examiner. Most CFIs know who the good ones are in their area.