Whether you can use an Oyster card immediately depends on how you acquired it. If you purchase a standard Oyster card from a vending machine or ticket office at a London Underground station, it is activated and ready for use the moment you add "Pay As You Go" credit or a Travelcard to it. You can tap in at the barriers and start your journey instantly. However, if you order a "Visitor Oyster Card" online to be delivered to your home address outside of London before your trip, it also arrives pre-activated with the credit you purchased, meaning you can use it as soon as you step off the plane or train. The only time there is a delay is if you add credit via the "Oyster Library" online portal; in this case, the credit may take up to 30 minutes to "sync" with the yellow card readers. For most travelers, buying a card at the station or simply using a contactless credit/debit card or mobile wallet (Apple/Google Pay) is the fastest way to travel, as contactless requires no card purchase or top-up at all.