Barry Salter wrote:

Is the North London Line covered by Oyster (other than the bits that are
duplicated by the District/Jubilee)? It's entirely possible to get between
the NLL and Central without going through a barrier.

According to the TfL Fares and Tickets leaflet, PAYG Oyster (formerly
PrePay) is only valid on the NLL between Richmond and Gunnersbury, and
between Canning Town and Stratford.

Hmm - I can't remember if the NLL is shown on the tube maps these days.
Wouldn't it be a heck of a lot easier to make the tube map show all and only
Oyster validity?