How hard do you want? Chern's question book is pretty good for OKAPS, but I think it tended to be a little easier than the Board questions.
Provision has questions that are very much like the boards.
If you want to be humbled, do the Mass Eye & Ear book. Those questions, on average, definitely harder. You'll learn a ton. If you feel like you need a kick in the rear to get going, that book should scare you into it.