See here (which I've just updated to talk about ISO; it didn't before):
[ul][li]Make RAR, 7z, ISO, etc. files open in Opus on double-click[/li][/ul]
Please see the Note about ISO files section, in case what you're planning to do won't work. Opus opens ISO files like archives, not as mounted drives. (For mounting ISO files, I recommend VirtualCloneDrive, which is free and works great with Opus.)