UKC

I just noticed that my OS app now shows routes in some other countries (check out Central Park)

I am certain that I saw a route on the OS App credited to OutdoorActive, though I can’t find it now.

What’s going on, how does this work?

Here's a question from their website 'help' section FAQ with it's answer:

Anyone can create and publish a route on there, social media style - but Outdoor Active are one of those 'outdoor route partners' that have loads of routes posted.  (But unlike a lot of the other 'partners' none of their routes seem to be within the UK - maybe that's part of a deal they've made with OutdoorActive.)

Thanks for that. Do you know if this is a recent thing?

I don't think 'recommended routes' created by various organisations being on there is particularly recent, but no idea at all whether or not Outdooractive being one of them is I'm afraid.

You can add routes abroad, whether or not OS have partnered with a topographic map provider by using the "Standard" Layer. Functionality will be more basic (No snap-to-path for example). I've added a few canoeing and walking routes in Sweden and France. 
You can also use the 3-D function anywhere, which I sometimes do whilst watching some geographically interesting programme on the telly and want to know more about the terrain around.