Call me stupid if you like, but I failed to see how to setup Google in Edge on Windows 10, this OpenSearch thing didn’t mean anything to me.
I did figure it and thought it’d be worth noting down:
First go to google.com (or .co.uk etc).
Then go to … -> Settings -> View advanced settings
In the “Search in the address bar with” dropdown choose “<Add new>”
http://www.google.com (or .co.uk etc) should be in the “Choose one” list.
Select it and click either “Add as default” or “Add”.
Personally I also turn off “Show search suggestions as I type” to avoid copy and paste text being sent to the search engine.