MOCAŠ
MB Enthusiast
A few years at least. Also the ticket machines at bus stops don't give change ...
Edit - here you go, it was 2003 this came in:
BBC NEWS | England | London | London's buses become cash free
Ah, right. I've seen the ticket machines around, but had no idea that pre-purchasing was mandatory.
Even so, that's a fairly small area, so the benefits of Oyster usage on buses would still be felt throughout the rest of London.