If you currently use pay as you go, travelling using contactless is a good alternative. You are charged adult-rate pay as you go fares, but you won't have to worry about running out of credit and could also benefit from daily and Monday to Sunday capping.
To benefit from capping you need to choose one card or other contactless device and use it for all your travel. We can't link the travel charges on your Oyster card with those on a contactless payment card.
You can use contactless at any time. You might want to use one if you forget your Oyster card or if it runs out of credit and you can't top it up.