I was browsing on the tfl website because it seems to me you would need a month's training to understand the options and likely charges with Oyster. I noticed the following under Travelcards:

"Travelcards are valid for travel on:

Tube, DLR, London Overground services. Your Travelcard must be valid for all the zones you travel through
Bus services within Greater London area (regardless of the zone(s) your Travelcard covers)
Tram services where your Travelcard includes Zone 3, 4, 5 or 6
National Rail services in London (excluding Heathrow Connect services between Hayes & Harlington and Heathrow, on Heathrow Express and on Southeastern high speed services between St Pancras International and Stratford International)
Scheduled Riverboat services at 1/3 off the normal fare. Just show your Travelcard (or Oyster card with Travelcard on it) at the time of travel "

Oddly this seems specific about where you can use a Travelcard for tube, buses and trams. As regards rail services it just says "in London". So if I buy a Zone 2 to 6 travelcard can I take the train to zone 1? Seems unlikely to me but that is how I read it.