Oxford is well connected by rail and coach from London. Trains run frequently from London Paddington and take about an hour. The Oxford Tube and X90 coach services run 24 hours a day from London Victoria and take about 90 minutes, often less outside rush hour.
If you are arriving at Heathrow Airport, the most convenient option is the Airline coach service, which runs directly to Oxford city centre in about 90 minutes. From Gatwick, National Express coaches connect to Oxford. Birmingham and other Midlands cities are also well served by direct trains.
Driving to Oxford is straightforward but parking in the city centre is extremely limited and expensive. The park-and-ride services on the outskirts are a much better option if you must bring a car. The city centre is largely pedestrianised and best explored on foot.