Have you installed cssu update on your N900? If not, you should. With CSSU instruction page there are instruction how to get the necessary Nokia repositorys and extras-repository (which should be abled before installing CSSU), to your Application Manager and it also installs automatically CSSU update when you go to that page with Micro-b browser and click in the instruction text the link for installing CSSU testing. And I think the Opera version I am using is actually in devel repository. You can install devel-repository from here http://repository.maemo.org just by clicking the install link if you open the page with micro-b browser.

The packet "rootsh" gives you ability to have sudo rights in console. After you install that you always in console first write "sudo gainroot" and it takes you to root. Then you do not need to wite sudo there for commands.

I think after doing these you should be able to install Opera. Not sure if you have to install extras-testing repository too. Noticed that I don't have it either it seems. Devel-repository should be only be enabled for certain downloads and then disable it.