Get the ASA IFR oral exam guide. Know everything in it and be able to spit the info out. You should have no problem passing the IFR oral after that. An examiner will have pet peeves that might go beyond that knowledge, but they will look at your knowledge level as an overall package and let you slide if you don't know their pet peeve item. Examiners like to walk away thinking you learned something and will keep at in until you do.