Dboy
Get a hold of a copy of the 'oral exam guide', some pretty good hints in there as to what the examiner may ask.
In short, know the aircraft and its sytems inside out, expect to have to describe various systems (electric/hyd) and maybe even draw them. Know the regulations, or at least the most important ones, if you can't remember during the oral exam you should be able to say, "I'm sorry i can't remember that but I know where to find it in the FARAIM", however if you need to use it for all the questions then expect to fail. If you have any weak area's on your written paper then again expect those area's to be questioned at some length.
Expect it to last anything from 30 minutes upwards to the whole day. A nightmare of which I have no desire to have to repeat! But if you prepare well and keep your cool then it should be relativley painless.