Oyster is a cashless payment system accepted thoughout public transport in Greater London with just a few exceptions. An Oyster card can store up to £90 in credit for pay-as-you-go, a travelcard season ticket or a bus/tram pass and a National Rail discount card. It can be used on buses, trams, the tube, DLR, National Rail services, Thames Clipper boats and the London Cable Car. You can buy them at tube stations, London Overground and Elizabeth line stations, some other National Rail stations, Oyster Ticket Stops (shops), at Oyster online and over the phone on 0343 222 1234. A £7 fee is required.
Each Oyster card contains a computer chip which communicates with Oyster readers (validators) when the card is placed flat on/near to the reader, usually called touching. The card does not actually have to touch the reader though, it can stay in it’s plastic sleeve or even in a handbag or wallet providing there are no other similar contactless cards nearby.
On buses you must touch in when boarding, while on trams you must touch in on a reader just before boarding. You do not touch out on either buses or trams because they operate on a flat fare. On the tube, DLR and National Rail you must touch in at the start of your journey and touch out at the end so that the correct fare can be charged. Where gates are in operation your card will open the gates if it is valid. If the gates are not in operation you must still touch in or out, either on the gates if they are locked open or on yellow validators which can be used either for entry or exit. A valid card will change the orange light to green along with one beep for an adult card or more than two beeps for a child card. If there is a problem then the light will change to red and you will get two beeps.