Just had this sent to me so I thought I would pass it on.
If you have a pay as you go Oyster card when visiting London you can get the Senior Railcard discount put on to your Oyster card (if it is already registered) by visiting a London Underground ticket office and presenting your Senior Railcard and Oyster card. You get 34% discount on off-peak pay as you go fares and daily caps, you can also buy discounted Off-Peak Day Travelcards to travel at any time on weekends and public holidays & after 09:30 Monday to Friday.