With Tab Stacking,  you may came to know that Opera tried to bring Firefox Panorama similar tab grouping in Opera, you need to enter Panorama through shortcut or by clicking Panorama icon then you can get overview  of tabs thumbnails, in Opera it’s different,  you can group tabs from with in tab bar.

Using Tab Stacking in Opera

It’s easy and fun to play with stacked tabs in Opera 11.  Drag one tab on top of another to form a group of tabs called a ‘stack’. Only one Tab is visible on the tab bar for a stack. When you hover mouse pointer over tab stack,  you will see thumbnails of tabs in that stack. 

tab stacking Opera

Each tab stack will have arrow icon on right side. By clicking that arrow icon, you can expand stack or collapse it  back again. Interesting thing is you can only expand one stack at a time, when you tried to expand another stack then other expanded stacks will be collapsed.