MIG wrote:

Anyway, just thougth I'd remind about the posters in SET trains which
don't mention OEPs or travelcards at all when encouraging people to
use Oyster. They talk about "use your Oyster card on our Greater
London Metro trains" or something similar.

It's similar on National Express although there are some additional posters
at stations about where the outer boundary is beyond which you can't use
Oyster. Nothing about OEPs at all.