Yup - you can use it on every rail service within Greater London (and a handful of places outside, like Watford Junction, but not HEX or HEC past Hayes). So Clapham Junction, East Croydon, Hampton Court - all available on PAYG now. However... You do have to touch out if you're travelling outside Greater London. Sometimes this is easy - you can even hop off one train, use an Oyster reader on the platform and hop back on the same train, but other times, the readers may be down a couple of flights of stairs. It's quite variable.

The only complication is if you have a season ticket loaded on your Oyster and want to use PAYG as a zone extension. If that's the case (and it will only be if you're staying a week or more) then you have to get an Oyster Extension Permit (OEP).