If you can take a course, obviously that will help considerably. It gives you confidence and great feedback and tips in the week or two right before taking the exam. I'm probably biased here, I'm a course examiner for the AAEM course (I'm sitting in a hotel room typing this and I have to be up in 4 hours to teach the course this morning).
If you can't do a course, that''s fine. I love the Okuda book, and I use cases in that book for my residents. I've run cases over skype with former residents preparing for boards using that book as well.
When I took oral boards, I studied by myself using that book. Was it ok? Sure, I passed. But it wasn't the best way to prepare. Will it be adequate anyways? Probably. But if you can't take a course, I'd suggest you find a fellow former resident to buddy up with via skype or facetime, and run cases for eachother. Not only will you learn from the cases you do, but you'll learn from your partners mistakes as well.