Visiting Oxford is often on most peoples itinerary whether they live in Dorchester, or just visiting.
The bus is often a good choice, and some details can be found on https://www.dorchester-on-thames.co.uk/information-2/travel/
Other options include driving (there are several park and rides, plus a large car park at the westgate shopping center).
Whilst the Ashmolean contains a vast array of exhibits, you may find a few related to the village. Particularly in the ‘England 400-600’ and ‘Rome’ exhibits