sort of, Mangetout described it upthread but basically you’ll never end up paying more than a daily “cap” that is equivalent to the cost of a day’s travel card.
i.e. Let’s say you don’t have an oyster card, you pay for a daily travel pass up front but it ends up a lovely day and you only take a single tube and a single bus journey. You walk the rest of the time so you’ll lose out a little.
If you have an Oyster card it would take for just those journeys but should you decide to take many more journeys you’ll stop being charged at the point where it hits that daily cap.
In short, if you have the oyster card you will always be certain that you are paying the very least amount for your travel, there is no downside.
Thanks so much for the responses. I’ve got the info I need and that is that I can load the card with a cash value, which wasn’t clear to me on the website.
We bought ours at Heathrow, and used them to head into the city center… we used it all week long, then when we got to Heathrow on the way home, we cashed it out and kept the card as a “souvenir.”
Had to top it up a few times, but it was just like using a Charlie Card in Boston, or the equivalent for the DC metro.