Fast trains (ICE, IC, RE) won't stop in Oppenheim. Visitors have to take the slow train (RB, Regionalbahn). 'RB44' runs from [wiki=3496da6c1db0a5223edb1fb4c415a154]Mainz[/wiki] via Oppenheim and [wiki=8ba2899ede00d981bb3faa9e58dd21b0]Worms[/wiki] to [wiki=bd2b145b873049a539012fea08bfa71a]Mannheim[/wiki] (or vice versa) every half an hour during normal hours on work days and once per hour on weekends. If you're coming in from nearby [wiki=4312d5c8cdda027420c474e2221abc34]Frankfurt[/wiki] airport then take a train to Mainz first and change there.
From Oppenheim's train station it is only a 5-10 minutes walk to the Old Town. (There is a city map across the street from the station building.)
The B9 connects Oppenheim to Mainz and Worms.