Each journey made with an Oyster card is individually monitored, and recorded. The points that were travelled to and from, and while Transport for London claim that the identity of the traveller is not associated with this data; they can and do divulge the data of named travellers upon request.
Supposedly there is an option to remain anonymous, but a good deal of searching for specifics was fruitless. Furthermore, TfL doesn’t appear to be a particularly reliable source on these matters.
