How are bus rides accounted for when using an Oyster card? London Underground stations have ticket barriers that scan the card on entry and exit, but using a bus the card is only scanned when boarding. How is the correct fare calculated in this case?
They do not need to - there is a fixed fare per bus journey (£1.50 at the time of writing), no matter how long you travel.
It gets a little bit more complicated if taking more than one journey - with the new 'Hopper Fare' you can make unlimited journeys (on busses) within one hour. A British transport enthusiast celebrated this by changing bus over twenty times in an hour.
There is a daily price-cap for bus travel too - no matter how many bus trips you take, you will be charged at most £4.50 (assuming no travel on other Oyster services).
The above applies for bus and tram (or a combination of the two) travel. Things are more complicated if you also take other services which are available on Oyster (tube, national rail, DLR, etc.)