In message , David
Cantrell writes

You can buy zone extension tickets at stations. OEPs will, as I
understand it, only be available at shops outside stations.

As Paul S has pointed out, the media briefing claims they will be
available at "most NR stations". How this turns out in practice is yet
to be seen, of course, but it would be the logical thing to do.

And in any case, "I've got a travelcard and a big enough PAYG balance,
therefore I've already got a zone extension ticket and it should Just
Work because Oyster is meant to make things simple and convenient".

It doesn't seem to me to be harder than having to remember, at present,
that the PAYG element on an Oyster can only be used on a very limited
number of NR lines. Or that a Freedom card can only be used after a
certain time on NR and only to certain stations... or having to remember
any of the other ticketing restrictions encountered in modern life.
--
Paul Terry