@MikeWh wrote about this on his site back in September.
www.oyster-rail.org.uk
One-Way System Woes – Oyster Fares Central

Finsbury Park
Access to the Underground from the National Rail subway is not permitted. Entry to the Underground is via the new Goodwin Street entrance (off Fonthill Road). Exit is via the Station Place entrance next to the National Rail entrance. The Oyster problems stem from the haphazzard way that GTR have gated some platforms but not others on the National Rail station. If using the northbound platforms (with gates) there shouldn’t be a problem because there is an OSI between National Rail and Underground. However, there are no gates on the southbound platforms so you have to use the validators in the subway. These are counted as part of the Underground station, so the OSI doesn’t work.