Previously only for National Rail and London Overground, from 2011 any of the cards below can be be linked to Oyster cards to reduce off-peak PAYG prices. Cards include:
Disabled Persons Railcard
HM Forces Railcard
The discount amounts to 34 per cent off off-peak pay as you go single fares on Tube, DLR, London Overground and National Rail (previously was just National Rail and London Overground) and also 34 per cent off off-peak Daily price caps on the Tube, DLR, London Overground and National Rail.
Worth registering for the savings over the year!
Worth pointing out as some people are getting confused: This is not the same as the current deal where you get 34% off the DAILY CAP, this is 34% off a single fare. So if you make journeys where you won't hit the discounted daily cap (around £4.20 currently) then you will still make a saving.