When commuting to, or commuting from the following two airports:
- Heathrow Airport
- Gatwick Airport
you can use the same Oyster card that you use in all London public transports.
For Heathrow it is quite obvious since you can actually take the tube to go to the center, but it also works for Gatwick Airport where you take the train instead. You can just touch-in and touch-out with the Oyster card and it is a few pounds cheaper than buying a train ticket.
To buy an Oyster at Gatwick : exit through Customs and buy an Oyster card at the Concierge outlets in Gatwick North Terminal or Gatwick South Terminal, at the Skybreak Service Centre in both terminals or at the Gatwick Airport train station.
When leaving London, at Gatwick Airport you can only get a refund for any unused pay as you go credit and the Oyster card deposit up to a combined value of £30. To do this, go to the rail ticket desk in the Onward Travel area in the South Terminal. This is the only place at Gatwick Airport where you can get a refund if you return your Oyster card.