Thank you all for your replies. My wife doesn't drive but we are willing to live on the suburbs of Oxford provided that she can take at least the bus to work which is at the city center. We are mainly interested in living near a city with certain amenities (restaurants, bars, shopping street, etc).

The idea of staying at Headington doesn't sound bad as it seems to be less than 30min from the city center by bus. From your replies, it seems that the 6th zone (Ickenham, Hillington or Rickmansworth) is the closest I can get by car to London before being stuck in the jam.

When I checked the commute from Didcot to London by train it wasn't faster (or cheaper) compared to Oxford. You reckon that Didcot would be more convenient though?