"docked a fiver"?
It's seven quid now: http://www.tfl.gov.uk/tickets/14873.aspx - it gets docked when you enter, and the difference is refunded when you exit.
Oyster fares started out quite simple, but they are now truly mind-boggling (thanks, Ken and Boris)
But hang on, that's not complicated enough. There are separate fares for Oyster journeys on National Rail, and also for Oyster journeys which are part Tube, part National Rail. See
And if that's not enough, try understanding Oyster Extension Permits.