There are two bus companies serving Oxford the Oxford Bus Company and Stagecoach. Both run frequent buses to London and the Oxford Bus Company also runs buses to Heathrow and Gatwick airports. There are also some coaches run by the National Express that run between Oxford and other parts of the UK.
Trains
The train station is in central Oxford and has frequent connections around the region to the coast and even Scotland. Check here for more information about rail travel including within Oxfordshire.