Every android app will have list of URLs that it can open. So you have to go to that app settings and tell that it should open in browser for the URLs and not in the app.
To do that go to Settings -> Apps -> scroll down to the app that you don't want URLs to open in -> Tap on 'Open by Default' and select always Ask. So next time you click on a url it won't open in any app directly it will ask you and you choose it to open in Opera.
