Videos are directly streaming onto my default player. Want to open in Opera so I can download it
You shouldn't need to uninstall anything. You just need to clear the default.

Go into your settings and the app manager. Find the default video player. (You'll probably have to change your view from "Downloaded" to "All" in order to find it. I expect it's called "Video" or similar.) In the settings for the app should be a button labeled "Clear defaults". Tap that and remove the existing default settings for the app.

Now when you go back into Opera and click a link it should prompt you again. Be careful about that "make it the default" button.