the best prep is to review "board style" scenarios. some people call these "old questions" either way- there are TONS of previously asked scenarios floating around. nobody can admit to really stockpiling or compiling this type of information though since the ABS prohibits reproducing the exam in any way shape or form- including writing stuff down from memory.
thats the best prep. theres only a finite amount of scenarios they can realistically ask.
osler helps because it helps you feel better about yourself when some of the other "less well-prepared" candidates get up to do their mock orals. ie- if THAT guy goes before me, I'll be a superstar!