I just had the same problem. Opera 42.0.2393.94. 1Password version is 6.5.3. Could not find the extension on Opera (https://addons.opera.com/en/search/?query=1password+agile&type=extensions).
Then went to: https://agilebits.com/onepassword/extensions/opera?beta=false and clicked on the Install button. I even tried the 'download the extension from its alternate location) link.
Then I got the "Extension installation failed. Attempted to downgrade extension".
What do I do?