True story from one of my ODs:
He had been studying for the MCAT with every intention of applying to med school. His sister was applying to Optometry school. On the day of her OAT, she asked him to drive her there and back. Not wanting to make two trips, he wanted to find something to do in the area during the 4-5 hours. What did he do? He decided, "Sure what the heck, I'll take the OAT with her. Good MCAT practice, right?"
Turns out he killed it and decided to apply to one optometry school. If he made it in, great, if not, back to med school plans. Well, he got accepted and has since been a very happy and excellent OD with a few of his own PPs (his sister did not get accepted to that school, which is the funny part 😛. But they now work together at one of the PPs.). In hindsight, he was very glad he got into optometry instead of medicine because he now realizes how much more suited he is to it.
Disclaimer: This is from back when many opt students were, in fact, med school rejects or whatever. They are cracking down on that now, because most people in his situation do not have such a happy ending. As such, I still recommend you shadow and figure out what you want to do. I just offered this scenario to show that it's not impossible to be a good OD if you were unsure at the start.
Oh and I used MCAT review books with OAT practice tests. Those + my bio degree, and I felt more than adequately prepared. 🙂 Good luck with whatever you decide!