In reply to Pedro50:
That is why you have to touch in and out.
When you go further than London you may need a ticket as well or instead. Worth knowing that if you want to go to some places just outside London you can save quite a bit if you can be bothered to get off the train at the last oyster station, touch out and then have a ticket from there.
Did this for Gatwick but apparently thats now got oyster.
Post edited at 10:40