In message
p.homeip.net, at
11:44:00 on Sun, 7 Mar 2010, Paul Cummins
remarked:
No. They are needed when travelling on National Rail and using PAYG
to go beyond the validity of an Oyster travelcard.

That's curious - I use PAYG all the time when I'm Oystering, and have never once
been informed that my Oyster is invalid because I don't have an OEP on the way back
to St Margarets from Waterloo.

And I've been gripped a few times - they just ask me where I'm going.

Do you have a Travelcard on your Oyster that's valid to not further than
Zone 3 (St Margarets is in Zone 4).

The OEP is only required as a form of "permit to travel" (ahead of
paying when you exit) for your trip from the relevant Travelcard limit
(eg Putney, the furthest Z2 station) to St Margarets.
--
Roland Perry