Trains run between Oxford to London's Paddington Station, which is about 30 minutes by London Underground to the centre of the City.
The journey takes about one hour and train run approximately every 30 minutes. For precise train times you should consult the railtrack travel guide page. This is a web form in which you simply need to enter `London' and `Oxford' as you two stations, and the day and time at which you wish to either leave or arrive.
The London Tourist Guide provides a great deal of information on London, Hotels, Night life, where to eat, travel etc. Another useful page is the London Guide, which contains details of how to visit the top ten attractions in London.