@ OP: If you think youre interested in OMFS then just go for it. Everyone on here will tell you that you need to pull a tooth or retract on a pec flap before you know that you wanna do it but I think you can know long before that.
I took the CBSE in the summer after my D1 year before we took our systems classes and did great on it without any knowledge from CVS/pulm/neuro/etc. classes. Even so, after taking the CBSE you realize that dental school doesnt provide much info of what will be tested on the CBSE anyways, so why wait?
If you really wanna get involved, start looking at sketchy micro. I say this because if you start looking into pathoma/first aid right now you wont really know what to study or how to study until you start UWorld when you really decide your study schedule. But for sketchy you just pure memorize it and youll be good. Start showing face at your school's OMFS clinic whether that be the hospital-based or DS based one, and then maybe a resident will ask you if you wanna help on a research project. Thats what happened to me and I got a couple publications out of it so it worked great.
Lastly, find a time to take the CBSE and study for 4-5 months for it and youll be good. Keep your class rank up and make sure you develop good hand skills. Controlling your handpiece to work with millimeters will translate to being naturally more comfortable with some aspects of surgery.
Goodluck and dont let kids who dont know what theyre talking about bring you down. Everybody whos been through the ringer and knows what it takes will tell you the same stuff, its never too early to start.