The ASA IFR Oral exam guide is very good, also make sure you can identify everything on an approach chart and low enroute chart ( symbols, intersections, freq's, mea, moca etc.). The examiner in our area likes to spend an hour just throwing questions at you in regard to the charts while you finger fly certain routes and scenarios on them. Be ready for a good portion of weather conditions and weather information services. On the flight just hold your altitudes and headings and RELAX. You'll do just fine. Good Luck!