You could take the OAT the spring of your junior year or fall of your senior year to get accepted to start right after you graduate. I think it's too early to start studying for it as a sophomore. If you have a long winter break (some people have all of January off), that would be a good block of time to prepare for the spring exam, for example.
You can always get the Lippincott OAT book (the skinny black one) now, if you just want to get an idea for what you'll have to look forward to. 😉