Work offline button helps the users to work offline when they do not want to connect to the internet .They can use this facility to view stored html or htm files.
To add work offline toggle button follow these instructions.
First open Opera.
Go to tools > appearance.
Go to the buttons tab and select the preferences buttons.
You will be able to see a lot of check boxes like small screen, work offline, and enable JavaScript etc.
Just click on the Work Offline check box and drag it to any of the bar, I would recommend the view bar. But you can also put it on the address bar as long as there is enough space. Now just leave the mouse button and you will find that you will have a Work Offline check box in that place .By default it will be unchecked.
Check this button when you want to work offline.