On Tue, 07 Sep 2010 13:13:46 +0100, Roy Badami
wrote:

Interesting, but I'm just not comfortable relying on it. If it were
just the risk of a PF that might be one thing, but there's always the
risk that they might prosecute.

So how would this situation be treated:
I and a colleague travelled from Euston to Richmond. I have PAYG, he
has a Z1-2 season.

Outward journey from Euston on Overground, via Willesden Jn. Touched
in at Euston, £1.30 PAYG for colleage indicated on exiting Richmond
station.

On the return journey, just missed Overground so caught the first
Waterloo train, changing at Vauxhall. Colleague charged £1.50 on
exit. As we didn't decided to use NR until the last minute, should my
colleague have tried to set an OEP for the return? Should the
barriers at Vauxhall NR have alerted someone to the absence of an OEP
and not have allowed him to leave?

Oyster FAQ is of little help...
You only need to set an Oyster Extension Permit when you start a
journey within the zones covered by your Travelcard and want to travel
outside those zones on National Rail

We didn't "want to travel" on NR, it was simply more convenient at the
time...