More about the benefits of an Oystercard and why you should by one in London here, but it’s a little known fact that it doesn’t cover transport all the way to ANY of London’s major tourist airports: Gatwick, Heathrow or Standsted. Instead, each airport offers its own express train.
For flying into London Gatwick, please note, it’s about two hours from London City Center, so please calculate an additional $70 USD/ £40 for two people for the Gatwick Express to Victoria Station. From there you can buy an Oystercard and get just about anywhere in London via the tube/ underground.
For flying into London Heathrow, it’s about an hour from London City Center. The Heathrow Express travels to Paddington station in 15 minutes for about $30 / £21 if you purchase in advance. To buy the Express ticket upon arrival into an airport it’s an additional $8 / £5 per ticket.
For flying into London Stansted, there are two public transport options to get near city center. For $14/ £9, a 55 minute bus ride will take you from the airport to Liverpool Street. For $27/ £18, a 45 minute Stansted Express train will get you from the airport to Liverpool Street.
NOTE: once you get to Liverpool street, you can use your Oystercard to get anywhere else on the tube, the overground, or the bus system.
If you’re traveling with a group or you don’t want to deal with inevitable crowds, a third option is a taxi from Stansted which costs between $120-150 (£80-£100), though I’ve never taken it. You’re better off downloading the Gett app and hailing a taxicab that way.
This is important so I’m saying it twice: buy your Oystercard in London. Do not get a travel card. Do not buy one-way tickets. And please look hard at the London Pass before subscribing. The Oystercard lets you travel like a Londoner through nearly all the zones and you can refill/ top up the card as you go.