Sharon :

The Oyster gives significant reductions on single cash fares. For example a bus fare in Central London is £1.50 cash now, whereas Oyster is only 80p off-peak, and £1 peak. Oyster does off-peak reductions you cannot do on normal cash fares. Likewise a suburban Underground journey is £3 cash but only £1 with Oyster.

Once you have made several journeys you get to the stage where it would have been more efficient to have the relevant one-day Travelcard. Oyster works this out and the "price cap" comes into play at that point, in fact at 50p beforehand.

The Oyster site makes this (rather convoluted) statement :

"During any 24-hour period from 0430 to 0430 the following day, you will never pay more than 50p less than the equivalent Day Travelcard price for all your Oyster single journeys in Zones 1-6".

You still have to "in-and-out" with it, but it is not charging any more. It works it all out based on the different fares for combinations of zones and what you actually have done.

From next year it seems Travelcards will disappear and it will be just Oyster. In fact Transport for London would like to do away with cash altogether, certainly with those kerbside bus ticket machines in Central London (very unreliable). Just too bad if you forget your Oyster one day I guess ......