There are frequent trains to London (Paddington or Marylebone) which take about one hour. Oxford also has good rail connections to other major cities. Oxford has two train stations:
Oxford station (OXF) is a short walk to the west of the city centre
Oxford Parkway (OXP) is on the northern edge of the city, but can easily be reached via bus services along the Banbury Road.
You can check times and buy rail tickets through National Rail Enquiries.
Fares depend not only on the length of your journey but also on the time of day when you would like to travel. Travelling at 'peak' times (before 9am and in the late afternoon) can be quite expensive. Travel at other times is referred to as 'off peak'.
You may be eligible for a railcard depending on your age and/or occupation. Railcards offer significant discounts on train fares and often the price of the card is paid off with your first journey. If you use the train for your commute to Oxford you may want to buy a season ticket (weekly, monthly or annual). You can find discount options for staff on the University's Travel pages.