It's very confusing when you travel from Paddington by train, you can go through the gates using your oyster card but you have to know if the gates of the destination you getting off at will allow you out using your oyster card.
7As far as you want if you stay on the circle line all day ... ;)– DavidPostillOct 10, 2015 at 22:29
6@David: That is, if only the Circle Line hadn't been uncircled back in 2009 ...– hmakholm left over MonicaOct 19, 2015 at 13:11
@HenningMakholm Really? I didn't know having not lived in London for the last 10 years or so. I suppose now you could go all the way to Hammersmith and then reverse direction?– DavidPostillOct 19, 2015 at 13:14
@David: It now goes from Hammersmith to Paddington, once around the circle (clockwise) and terminates at Edgware Road. Then back again. No direct trains between Bayswater and Baker Street, other than the long way around.– hmakholm left over MonicaOct 19, 2015 at 13:19
6You can travel around the world with your Oyster card. Put it in your pocket...– CGCampbellNov 15, 2015 at 21:26
You can travel on most public transportation in London with your Oyster Card - Bus, Tube, tram, DLR (Docklands Light Railway), London Overground, and Thames Clipper boats. You can even use it on the Emirates Air Line.
The Oyster Card gives you a wide area of travel. The Tube has 9 zones. As long as you start and finish your journey within these 9 zones, you can use your Oyster Card. You can find more information here: https://tfl.gov.uk/fares-and-payments/oyster/using-oyster/where-you-can-use-oyster
You mentioned Paddington - You cannot use your Oyster Card on the Heathrow Express which travels between Paddington and Heathrow Airport. But, you can easily use it at Paddington on the Tube (Bakerloo, Circle, District, and Hammersmith & City lines).
Starting in September 2015, you can easily touch in and touch out on the Thames Clipper boats with your Oyster Card to pay for your river travel. This makes travel on the river Thames easy. Here is a map of the routes it takes: http://www.thamesclippers.com/route-time-table/book-now
+1, You answered 45 minutes after the first person who answered, but included a lot more detail. That's fine. Oct 11, 2015 at 0:25
You can go anywhere in zones 1 to 9 (which excludes travelling to Heathrow airport by Heathrow Express or Heathrow Connect). On national rail from Paddington, that's basically as far as West Drayton, Hayes & Harlington, or Greenford.
You can see a full explanation and a map of where you can travel to (including from other mainline stations) on the TFL website: https://tfl.gov.uk/fares-and-payments/oyster/using-oyster/where-you-can-use-oyster
+1 for the link to the list, as you can now use Oyster to Watford Junction, Gatwick airport, Hertford East and other places outside zone 9. Apr 28, 2016 at 11:37
Generally for transport that runs on rails (train, tube, DLR, Trams) you want the "London's rail and tube services" map which shows the oyster zones and the edge of the pay as you go areas. http://content.tfl.gov.uk/london-rail-and-tube-services-map.pdf .
Note that there are a few areas that are outside of any travelcard zone but where pay as you go can be used. In some of those areas both contactless PAYG and Oyster can be used, in others only contactless.
Oyster can also be used on London buses, but I don't know how far out you have to get before the buses stop being London buses. I'm sure there used to be a map but if there did I can't find it now.