for the benefit of others who may read the thread
a) you just beep the card on the card reader on the bus / at the station - it's the same reader whether it's oyster or contactless card.
b) with buses, you just beep on boarding - the fare is the same however many journeys you make
c) with trains / underground / DLR, you need to beep on entering and leaving, so it knows what fare to charge. If you fail to register out, I think it will charge you the maximum for a London journey. Remember this even if there's no ticket barriers.
d) there are some journeys by rail (usually those involving one of the orbital 'overground' routes) where with Oyster, you're supposed to beep on a pink card reader where you change trains to prove you've stayed in zone 2 and not gone through central london.
One thing that may be worth mentioning though, Oyster has a 'daily price cap' - i.e. .when you've reached the equivalent cost of a one day travelcard / one day bus pass for the travel you've made so far that day, you stop paying any more (unless you go into a further zone) - to the best of my knowledge, contactless bank cards don't do that so if you're doing a lot of travel on one day, it could g.et expensive.