It's a book. You can buy it directly from orgoman.com, and I sometimes see it being sold on Ebay.
However, I would strongly recommend to get some REVIEW books first (such as Kaplan). The OAT Destroyer is pretty much a big book of practice questions, so I would recommend reviewing the subject material before you dive into the Destroyer.
I'm not entirely sure, but I've seen many people study for the OAT using MCAT materials. I suppose they cover the same material, though. Just be sure to use practice exams specifically designed for the OAT. The OAT and MCAT's test formats are entirely different.