Here are a few more data points for you guys! I just wrote the OAT today, and I took the last acheiver test on Monday.
Achiever 1, 2, 3, OAT
QR 330, 330, 330, 360
RC 340, 310, 260, 360
Bio 340, 360, 330, 400
GC 340, 310, 320, 360
OC 340, 330, 370, 390
Phy 330, 320, 320, 360
TS 350, 340, 350, 390
AA 340, 330, 320, 370
Study Resources: Kaplan MCAT, OAT, and DAT books, as well as OAT Acheiver and khanacademy/chad's videos/freelance teacher videos.
OAT Acheiver is without a doubt more difficult than the real OAT. However it is important not to view the Acheiver exams as simply practice exams; they are much more than that. When used correctly, the Acheiver exams can be an invaluable tool to diagnose areas of weakness in your understanding of concepts and your general knowledge base. Go through the solutions to each and every question whether you got them correct or incorrect. If you find that you are consistently getting certain concepts/question incorrect, or are getting questions correct for the wrong reasons, immediately turn to your other resources and go over the material that seems to be giving you trouble!
I found that the QR section on the real OAT was actually quite similar to the Acheiver QR tests, but I found that the RC on the real OAT was MUCH easier than the Acheiver tests. The passages were more organized, easier to read and altogether more interesting. The questions were a bit more straight forward and often came sequentially so it was easy to locate the answers in the passages. As for the Bio, GC, Phys, and Ochem sections on the real thing, they seemed to be harder than that ADA exam but easier than the Acheiver exam.