There is no magic fairy dust protecting Macs - Dai Zovi, security researcher and co-author of The Mac Hacker's Handbook.
I just upgraded Opera by downloading the installer via Secunia, and Opera took over as default web browser – I don’t remember seeing a prompt for permission to make such a change.
Not long ago, I installed Opera as a part of my regular testing processes. No, Opera did not become my default web browser. Nevertheless, just like any other web browsers or applications, you need to be careful while you are installing the applications. In particular, you have to read the instructions and dialog box carefully to make the relevant choices. I’ve removed Opera after done testing it. My current default web browser is Internet Explorer 9. Both Chrome and Firefox are installed in my PC for evaluation purposes. I mostly use Internet Explorer 9 these days.
There is an option to install (or not if don`t want) Opera as the default browser ...