Wander, you buy a bus ticket at a tube station - go figure. You used to be able to buy a ticket at the machine by the bus stops at the RAF Club, now you have to walk the length of Piccadilly to Green Park.
Of course pensioners don't need Oyster cards or tickets to use the buses provided you have a bus pass (from age 60). ot sure is a European bus pass would cut it.
On travelling with a bus pass I can say it is brilliant. No longer do you have to wait for your bus and pay one exorbitant fare. You can jump on any bus going in your general direction, jump off before it leaves your route and pop on the next one going the right way.