Snags...the voice of reason !
Why on earth would anyone want to go to all the bother of buying an Oyster card instead of using a One Day travel card !
You can go anywhere you like, as many times as you like with this card.
Where is your destination "on the other side of London" We are always happy to help here on AB !
To start with, the Tube runs around all the Terminals at Heathrow, and will take you anywhere you like in outer and inner London, including ALL the mainline train stations.
Please note that the Heathrow Express fast route, into Paddington is not included on a One-day Travel Card. See this link ::
https://www.heathrowexpress.com/tickets-deals/railcards
The Heathrow Express is fast, only 15 mins into central London, but its very expensive. The Tube takes much longer but will be included in your One-Day Travel Card. It has the advantage of being capable of taking a huge distance, without changing if your destination is on the Piccadilly Line.