I have a question about how OEPs work when you board the train within the final zone of your TravelCard's validity.
As we know, if you have a TravelCard on your Oyster and want to travel beyond the TravelCard's zones on National Rail and pay for it using PAYG cash on the Oyster card, you must put an "Oyster Extension Permit" (OEP) onto the card before you start your journey, so that any ticket inspector who checks your card knows that you are intending to trevael out of zone.
So, if you are have a zone 1 & 2 TravelCard on Oyster and are travelling from central London (Charing Cross, say) to Woolwich (which is in zone 4), before you get on the train at Charing Cross you must visit a ticket machine to put an OEP on your card, and then touch out at Woolwich Arsenal station to get charged the correct add-on PAYG fare.
That's fine. However, what happens if I have a Z1/2 card and board somewhere in zone 2? Do I still need to put the OEP on, or will that journey all be handled on PAYG?
Presumably if I board at Greenwich railway station, which is in both zones 2 and 3, no OEP is needed - the journey will be taken as starting in zone 3, so will be entirely on PAYG. What if I board at Deptford, which if only in zone 2? Do I need an OEP for that journey because the the TravelCard covers me from Deptford to the boundary of zone 2, and the PAYG thereafter?