Thanks for the explanations and references. I think I (kind of)
understand now. I'll go off and read the Oyster terms and conditions now...

If a journey is part NR, then an OEP is still required. OEPs are not
required on London Overground network (which these days for ticketing
purposes acts as if it's part of the LU network - though of course NR
ticketing validities continue to apply, IYSWIM - the two run side by
side).

But if on an NR+tube journey the NR part of the journey is wholy within
the area covered by the travelcard, and it's only the tube part of the
journey that goes outside, I'm thinking maybe an OEP isn't necessary?
Or does the fact that NR was involved at all mean an OEP is required?

It's a screw up on the part of the RPIs who issued the PF. Whilst the
rules do technically state that a passenger should touch-in when using
an Oyster card even when it's loaded with a Travelcard, they don't
make out it's a do-or-die situation (I think the wording is "should"
rather than "must" - and only if there's an Oyster validator at the
starting station.)

Oh, I'd always (mis)understood that one *didn't* need to touch in with a
travelcard, e.g. when boarding a bendybus? Or has this always been
required?

Is it/has it always been required on DLR?

-roy