If oil gets on the parts, it will have a great chance in ruining the coating.
From how I would prep the parts to be coated would be to sand the part down (by hand, with blaster, or media/soda/nut shells when applicable) then thoroughly wash the parts with a cleaner that leaves no residue behind following up with clean hot water and a lint free cloth. After its wiped down, toss it in an oven to bake anything that I might have missed off. Let it cool, prepare the coat process. That is how I would go about it, however I am not in the business of powder coating and it may not even be correct to follow.