In message , David
Cantrell writes

They're a cunning plan by the TOCs to make PAYG a pain in the arse to
use even when they, strictly speaking, accept PAYG.

Unless I've entirely misunderstood, you don't need an OEP if you are
intending to travel using a PAYG Oyster.

It is only needed if you have a season/travelcard on your Oyster and you
want to use PAYG in order to pay for an out-of-zone journey.
--
Paul Terry