Why in the world did you pay for the car but have the title in her name? I hate to tell you this, but you bought your ex-girlfriend a car unless you have something in writing indicating that the car was supposed to be yours.
In Ohio, ownership is conveyed by title. When you had the title prepared to be just your girlfriend's name at the time, it legally became exclusively hers (next time, at least have your name on the title, too.)
You could attempt to sue her for unjust enrichment by getting a car she didn't pay for, but she could claim it was a gift. After all, why else would you pay for a car and put it in her name?
You may want a second opinion, but in my opinion, it's gone.