Some of my exams are oral, but I usually have some written test before and that helps me because the professor can actually see that I know the subject even if I can't talk about it fluently. Maybe try to talk to you examinators about it? Either beforehand or just before starting. That should give you some peace of mind, and at least they will know (if they trust you, I hope so) that your aren't stuttering because you don't know anything.