Eating out in Oxford is a real pleasure. The size of the city means you don't have to trek miles to find a good meal and this is fortunate because parking - as ever - is an issue. If you're staying centrally then you have lots of really excellent restaurants in easy walking distance.
We English eat earlier than in countries like Spain. Lunch at 1pm and dinner at 7.30 or 8pm are the norm. Do look out for "early dining" or "pre-theatre" offers. These are promotional initiatives to get you in early and can prove very good value.
It's worth knowing that tipping in the UK is 10%. You can give more but that is not expected in the way that it is in the States. It's often possible to add the tip when paying by credit card, but not all restaurants have this facility. In any case if you tip in cash then you know the money is going directly to the serving staff and not the management.
Generally service in British restaurants is not as good as on the Continent or in the States. I travel a lot for my work and I enjoy going out to eat more in just about every other country I know than in Britain. There has been a long-term attitude problem. The British traditionally have not taken pride in service and, perversely, those waiters who did consider their job important tended to behave scornfully towards customers, as is still the case in some Paris restaurants. My advice is - if your order is late arriving of if the waitstaff are chatting at the counter and ignoring you, or if items are forgotten or the bill takes forever to come or there is a general surliness - don't take it personally. We all have to put up with it. The one thing I don't put up with is a waiter leaning across one guest to serve another at the table. This is a bad habit on the increase. Please join me in pointing out politely that to lean across is rude.
This said, waiting a table is improving in the UK. The influx of keen trainees from the EU has made a difference, although it has also meant that making yourself understood in English is getting less easy.