I can't really say that I found any great comprehensive study guides when I was preparing to take the OAT. However, I think that the Kaplan MCAT study guide gives a great biology review. As for the other sections (Physics, Gen. Chem, and Org. Chem.), I used class notes and answered questions from the back of chapters in my textbooks. I used the OAT booklet as a guide for which subjects/chapters in my textbooks to review. I completed word problems in GRE review books to help me become a little more efficient at working quantitative reasoning questions. This may not be the best way for everyone to study, but it worked for me.