can I use an oyster card if buying a train ticket from london to oxford ?
william, I believe the Oyster Card is only valid for the Tube, Docklands Light Rail and perhaps some Buses, but I can't remember the details. I'm sure one of the others will be able to offer more specific information.
Ken, Oyster PAYG can now be used for suburban rail in London. However, Oxford is way out of London - you definitely cannot use pay as you go Oyster to go there. If, however, you have a weekly London Travelcard that covers several zones (1-4 or all zone) you can get a worthwhile discount by getting a fare from the edge of your Travelcard validity to Oxford. These fares aren't sold from machines - you will have to go to the ticket office and speak to a human. (This is only worth it if you have a multi-day Travelcard that includes the day you want to go to Oxford, don't get a Travelcard just for this.)
If convenient you could use this confusingly named bus service to Oxford. As a rule train travel is quicker and (touchwood) not subject to the delays that road travel can bring. http://www.oxfordtube.com/