In message
,
Railist writes

My real life example last week: found myself in Barking (had travelled
there from Chingford by bus).

Barking is in Zone 4, at the time I had Zone 1 - 3 travelcard.
When I touched in, on one platform were Westbound District line trains
and on the adjacent platform was a c2c train.
So I would need an OEP for one service bit not the other.

No. You don't need an OEP when travelling back into your zones - the
fare is handled by Oyster in the normal way (PAYG for the first bit and
travelcard for the rest).

You only need an OEP when travelling OUT of your zones by NR.
--
Paul Terry