Apparently it is very simple, but I see that some people have a headache when it comes to set Opera default browser.
Compared to Firefox, Google Chrome, Safari and Internet Explorer, The opera has a little hidden option “Use Opera as default web browser“, and who did not set “default browser” for the first time and had the option “Do not show this dialog again” The tick, you will have to enter the opera menu to set the browser default.

How we set Oprea, default web browser.
1. Opera Menu (left up) > Settings > Preferences (or Ctrl+F12 direct)

2. In the panel “Preferences” We go to the left menu to “Programs”, and on the right side bifam “Check if Opera is defalt browser on startup“.

Click OK.
When we reopen the browser we will be asked if we want to set the work as a default browser. Click Yes.
 
			