I don't know if Air Berlin offer special rail ticket fares. Budget airlines generally don't, but I would look on the Air Berlin website.
I did a search on www.bahn.de, and it looks as if Frankfurt Airport-Oberwesel can be done directly with a Regional Express (RE) train which runs every two hours, and lasts about 1 hour 15 minutes. RE trains don't usually offer discount fares, and the single fare on the day was quoted as 15.30 Euros. It may be worth looking on the site for discount fares, but the standard fare isn't very high, and if you get a cheap advance fare you will have to buy a new full-price ticket if your flight is delayed and you miss the train you booked. So I wouldn't bother booking in advance for this trip.