Hi, I would say taking physiology is far more useful for the oat than anatomy. I realize people may disagree, but here is my reasoning: anatomy isn't too difficult to study for as you are prepping for the test (even if you haven't taken a course in it).
Physiology, on the other hand, has quite a bit of material, some of which I can almost assure you will be on your oat. Things like action potentials, the digestion pathway, hormones, etc. are very difficult to understand without actually taking a class on them. Anatomy is definitely useful too, but there just isn't nearly as much complex material from anatomy that you'd need to know for the oat (you'd want to know things like the chorion, allantois, important blood vessels/features, etc., most of the muscle material would be covered in physiology too so you wouldn't have to worry so much about that). Again, if you are good at biology, you could teach yourself most of the oat level anatomy you didn't know yet pretty quickly.