http://help.opera.com/Linux/12.10/en/offline.html
"Work offline
If you want to catch up on reading webpages and mail while you are not connected, for example, while travelling, or if you want to cut down on internet costs, you can use Opera's offline mode. Follow the steps below:
Go to Settings > Preferences > Advanced > History and make sure that Cache is "On", and uncheck "Empty on Exit". This ensures that the mail and webpages you have open will be available.
Select Settings > Work Offline.
To return to online mode, select this menu option and uncheck "Work Offline".
If you are having problems, in the history preferences:
Set "Check images" and "Check Documents" to "Never".
Change "Memory cache" to 60mb, and "Disk cache" to 400mb.
See settings in the Storage topic."
This may help.