Where can I use Oyster or Contactless.

Not quite true, but close enough for most times the question is asked, the answer can be summarised as “anywhere in London”.
For a more accurate answer, refer to this map
https://www.tfl.gov.uk/cdn/static/cm...rvices-map.pdf

Note. Oyster is accepted on all trains / tubes / buses in London, except.

• Not accepted on Heathrow Express
• Not accepted on Heathrow Connect services on the section of the journey into the airport – but is valid between intermediate stations on Connect before reaching the airport. This is because this piece of rail line was privately funded by the owners of the airport, and they seek to recoup their investment by charging premium fares.
• Not accepted on the “SouthEastern High Speed” service between St Pancras and Stratford International. Because someone in the Department of Transport said no, and nobody else in the UK can understand why.
• Not accepted on sightseeing tour buses. These are not part of the public transport infrastructure within London.
• Contactless is not accepted on the small number of “heritage routemaster” buses which operate on part of bus route 15 between Trafalgar Square and Tower Hill. Oyster is accepted. The restriction on Contactless is because the ticket equipment used by the conductors hasn’t been upgraded to handle contactless.

Other buses on route 15 (modern ones, not heritage routemasters) are OK.