I have noticed that when being in offline mode (the computer is not connected to the Internet), Developer Tools (AKA Dragonfly) in Opera are not accessible. When choosing them from the menu, Opera thinks for a while then displays a connection error.
Is there any way to use Developer Tools without being connected to Internet?
As stated in Dragonfly documentation, it should be cached in persistent cache.
But if it is not, then you could try to download actual release of dragonfly to your computer from https://dragonfly.opera.com/app/zips/, unzip and add it's url to opera:config#DeveloperTools|DeveloperToolsURL (that's addres to paste in location bar, but SO doesn't want to output it as link because it has not http in front, I think).
Here is link: http://my.opera.com/haavard/blog/2008/05/07/opera-dragonfly-is-here-and-it-works-offline with article describing that