Thanks a lot Paul C. for posting up the info. I always assumed for
Camden Town and Camden Road to be a valid OSI but the following
example shows a fare anomaly.

Tufnell Park - Hackney Central

Oyster PAYG Peak / Off Peak
up till 1.1.10 : £2.20 / £1.60
from 2.1.10 : £2.30 / £1.80

This clearly shows that the route is priced via Zone 1, but it is far
easier and more direct to go via Northern Line to Camden Town, walk to
Camden Road ( valid OSI) and catch London Overground to Hackney
Central ? Yet neither of the TFL Fare Finder searches show the cheaper
( non Zone 1) alternative fare ? You can go from Wembley Central to
Hackney Central via LU to Willesden Junction and LO from there. An
alternative cheaper fare is available even though Wembley Central is
further out in terms of distance to Tufnell Pk. ???!!!! Is there an
error here ? Folk in NW6 seem hard done by !