Sent an email to the Oyster help desk and received this response:
"Oyster cards do not expire or get inactive just because you have not used them for a long time.

They might stop working for a series of other reasons, for example when they're damaged (bent, scratched, weare and teare etc).

Unless you've stopped them yourself by reporting them as lost or stolen, your Oyster cards will be good to use.

Our website is currently setup to only accept United Kingdom (UK) registered addresses and payment cards. This was introduced as a means of combating fraud. Although this may change in the future, you won’t be able to make use of the online account feature at the moment. You can top-up at any staffed London Underground station, Oyster Ticket Stops or Travel Information Centre, when you’re next in London."