There are no direct trains that night. 7 minutes is normally enough to catch a train at Mainz station - or DB would not offer the connection on its website. If you are extremely fortunate and leave FRA on the 20:59, your train to Mainz arrives at platform 1, the closest platform to the MRB train platform for trains to Oberwesel, and you'll be in Oberwesel 1.5 hours after leaving FRA.
But your problem is getting out of the airport promptly - flight delays, baggage, customs, passport control, ATM, walking to the train station, tickets.... lots of variables. I would just stay in Mainz instead in case you do have a delay at FRA. There are relatively slim connections to Oberwesel that night but direct trains to Mainz every half hour. (20:59, 21:29, 21:59, 22:29) and it takes only 25 minutes to get to Mainz.