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You don't need to buy an Oyster card if you are happy to pay directly with a contactless bank card, which you touch to the reader at the station gates at Heathrow, and a standalone reader at the final destination on the DLR. Canadian cards should work fine.
See the
short video from TfL on this page for how to use contactless cards to pay for journeys on public transport in London.
Alas not every contactless card issued outside the UK works works yet -
see info. Try it and see.