By taxi
Oxford has both metered taxis which can be flagged down from the street or taken from taxi stands located around the city as well as 'minicabs' which must be ordered by phone. Meter taxis are quite pricey but are convenient for short hops if travelling in a big group. Minicabs are much cheaper for long-distance journeys - the fare should be agreed over the phone when booking or should be bargained with the driver - never get in a minicab without agreeing the price.
By car
Driving in central Oxford is NOT recommended. Traffic is heavy, the one-way system is very confusing, the streets are often very narrow and have restrictions, and parking is VERY expensive. You're best off using the park & ride system or forgetting the car and coming in by public transport. If you have a motorcycle or a scooter things are a little easier.
By bike
The preferred mode of transport for the university student is the bicycle and like Amsterdam, Copenhagen or Beijing, you'll see hundreds of them. Most trains into Oxford will allow bicycles to be carried for free. Unfortunately cycle lanes are sporadic at best so often you'll be sharing the road with other motorists. The best option is to follow the locals as they know what they are doing, although remember that it's illegal for cyclists to run red lights although many do and you should use lights at night. Bike parking is available everywhere, but make sure you get a strong lock as bike-theft is a possibility.