I'll be arriving at Heathrow. I plan to get an Oyster card for the week but I hear that this won't work to get me from terminal 5 to London. Will it be clear when I get there what kind of fare to buy to get myself to London?
You can take the Heathrow Express (not covered by the Oyster Card although there may be a discount) directly from Terminal 5. We do this even when we have an Oyster Card (which is great value for the busy tourist).
You may use an Oyster card to ride the tube from terminal 5 to Central London, and vice versa. The card will not work for the Heathrow Connect or Express trains as they are a separate service.
The Heathrow Express is much more expensive than the Tube (£20 or about $30 at today's exchange rate, compared to about £5). It's been a while since I arrived into T5 and needed to use the Tube, but unless something's changed, there was a Tube station for the Piccadilly line in the lowest level of T5. I don't see why you couldn't use an Oyster card; it's Zone 6 going to Zone 1 if you're headed to central London.
This is great information. Thank you!