Depends on your standard required. Not really much to recommend in the city. The Oxford Kitchen in Summertown got good reviews a few years ago, no idea if they have maintained their standards.
The Oxford Kitchen They claim a 1 star Michelin rating but this can be deceiving. (Eaten at a 1 star place that was appalling!)
If you wanted a good but inexpensive lunch, then I would head for the Greek places in the Covered Market. Huge portions and very cheap.
Once upon a time you could get a decent meal at
Browns Restaurant Bistro but, again, haven't been there in years.
We tend to stick to places outside Oxford.
The White Hart at Fyfield (off the A420) is excellent and is Michelin listed.
If you really want to push the boat out, Le Manoir is the best but, you'll need a very, very healthy bank balance to go there!