Oxford is easily reachable by public transports.

From London:
There are trains departing from Paddington and Marylebone every 20 minutes.

From Brussels and Paris:
There are cheaper Eurostar tickets (if booked in advance) to get to London, St. Pancras.
Then from St. Pancras (Kings Cross rail station), you can take the metro, stop to Paddington station, and get the train to Oxford. 
Alternatively, you can get the bus (called Oxford Tube) from Marble Arch to Oxford.