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.
A single fare is £4. Children five to eighteen will travel at half-price. The bus accepts concessionary travel passes.
More information and timetables can be found on the Going Forward Buses website.