Read the document here.
A summary of the information. I recomend reading the full document.
Via diamondgeezer's fantastic blog on London.
If you don't travel into or our of London using National Rail, you can ignore this post and head back to the homepage.
Here's a summary of Oyster Extension Permits:
You can set your permit at any Tube or London Overground station ticket office or touchscreen ticket machine, Oyster Ticket Stops, some National Rail ticket offices or self-service ticket machines.
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.
Only set your permit to your card just before you make your extension journey.
You must have at least £1.50 pay as you go balance on your Oyster card to set one.
When touching out, the permit will be cleared from your Oyster card and the fare for the extension journey will be deducted from your pay as you go balance.
If you travel beyond the zones covered by your Travelcard without an Oyster Extension Permit you may be liable to a penalty fare or prosecution.
It should be noted that an usused Oyster Extension Permit will remain on the card until used.
I feel that many London travellers are unaware of what Oyster Extension Permits are. Hopefully the links above help.
diamondgeezer's blog is full of wonderful information on London, by the way.
I often become hooked on the blog when considering where I want to go walking next.
My last walk was from Clapham North to Morden. I want to walk to Erith but the snow is hindering this.