Thames Travel operates a weekday service between Didcot and Culham Science Centre which calls at Long Wittenham. See the Route 95 Timetable.
Going Forward Buses of Goring operates a minibus service on Mondays and Wednesdays from Wallingford to Didcot via Long Wittenham. See the link below for details.
More information and timetables can be found on the Going Forward Buses website.