The Oyster card is a blue plastic smartcard introduced in 2003 to replace paper tickets on London transport. It works by touching it on a yellow reader at the start and end of a journey.
You probably do not need one
This is the most useful thing a visitor can know. Since 2014, London has accepted contactless bank cards and phones directly on every reader, at exactly the same fares as Oyster, with exactly the same capping. Tapping your own card or Apple Pay works everywhere Oyster does.
Buying an Oyster means paying a £7 deposit, topping it up, and reclaiming any balance before you leave. Unless you need a specific railcard discount or a child fare, using the card in your pocket is simpler.
Capping
The feature that makes London’s system unusually forgiving. Fares are automatically capped daily and weekly: once you have made enough journeys to reach the cost of a day pass, everything after that is free.
You never need to work out in advance whether a travelcard is worth it. Just tap for every journey and the system charges you the cheapest applicable total.
One condition: weekly capping only works if you use the same contactless card or device every time. Mixing your phone and your physical card — even the same account — breaks the week into separate journeys and costs you more.
Tap in and tap out
On the Tube, Overground, DLR and rail you must tap both at the start and the end. Failing to tap out charges the maximum fare for that journey, which is substantial.
On buses and trams you tap only on the way in. There is no reader at the exit, and hunting for one marks you out immediately.
Where it works
The Tube, buses, trams, DLR, the Overground, the Elizabeth line, the Emirates Air Line cable car, most National Rail services within London, and Thames Clipper river buses. It covers essentially everything within Greater London.
It does not work on long-distance rail beyond the London zones — a train to Oxford or Brighton needs a normal ticket.
The name
Chosen by a branding agency in 1998 and never satisfactorily explained. The official gloss is that oysters are hard-shelled and protective, containing something valuable, with a nod to the phrase “the world is your oyster” and to London’s historic oyster beds in the Thames estuary. In practice it was picked because it was short, memorable and available.