One big question mark over Oyster PAYG acceptance on London Overground
has been how fares north of Hatch End - i.e. outside the 6 London
zones - will be handled.

One incredibly obvious place to look, which I've only just thought of
(d'oh!), is of course the TfL "Single fare finder" he
http://www.tfl.gov.uk/tfl/tickets/faresandtickets/2007/farefinder/

This doesn't provide any information as to whether there will be any
daily capping for such journeys, nor does it provide a fare table, but
it does provide the Oyster fares that will be charged (though not the
cash fares as "Special cash fares apply to selected London Overground
journeys").

A Watford Jn to Hatch End Oyster PAYG fare is thus £2 peak, £1 off-
peak (bearing in mind that Oyster PAYG peak times are M-F 7am to 7pm).
Meanwhile the National Rail Journey Planner states that an SDS is
£3.10, a CDS £3.00, a SDR £4.80 and a CDR £3.10.

Meanwhile a Watford Jn to Euston Oyster PAYG fare is £5.50 peak, £3.00
off-peak. Meanwhile the NR JP states that an SDS is £7.20, there is no
CDS, an SDR is 11.70 and a CDR is £7.40. Of course the printed tickets
would mean one could travel on the fast London Midland trains straight
into Euston, whilst Oyster PAYG would not be valid from Watford Jn on
these trains.

Presumably Oyster PAYG users will benefit from capping within the
London zones 1-6, so I'd guess that if a passenger reaches a cap using
their their Oyster card within zones 1-6 then if they travel from/to
Watford Jn they'd simply be charged the £2 peak/ £1 off-peak to cover
them from Hatch End (last station in zone 6) up to Watford Jn (or vice
versa).

This is all just speculation, so I might have it very wrong! Anyway
the TfL single fares finder is there for people to fiddle around with.