I tried one of the ticket machines at Euston main line today and it
didn't have an OEP option on there but it's certainly very easy to do
on an Underground ticket machine so I did that this afternoon at
Cannon Street and I touched out no problem at Charlton and it charged
me the correct fare of £1.40. On my way back though I was going to
Greenwich and the Oyster readers on the platform which gives step free
interchange to the DLR were both out of service so I had to go
downstairs to find one.

I think if these OEPs are going to work then there's going to have to
be some publicity (strategically placed posters etc.) as I know very
few Oyster Travelcard holders who know anything about them.