A bit of reassurance, one less thing for you to worry about: Although I had done lots of reading on this website prior to arrival, when I actually arrived at Heathrow and ready to take the tube out on the Piccadilly line,I was a little shy about how to get the oyster card. I swallowed any hesitancy, walked up to to London transit workers at the tube stop in Heathrow, and simply asked for help. The man walked me over to the machine, helped me with Purchasing an Oyster card using the credit card machines. He was super nice, asking how long we would be in London, where we intended to use the tube , number of days, super nice, amazing customer service. Couldn't have been more helpful. During our vacation, we had multiple scenarios at different tube stops where we simply asked the workers and they were always extra helpful.
Enjoy your trip!