Minimize the page
There was an option in Opera 12 to minimize the page (by pressing '4' on the keyboard or selecting "Minimize the page" from the context menu). Is it possible to do the same in Opera 22? Or maybe there is an extension with this feature?
Not available as far as I know (though maybe it's out there somewhere via extension, or will be in the future , but not sure why you need it. You can pin the tab to save tab bar space. Or just go to another tab.
However, maybe the Chrome extension, "One Tab" would help you even more (if it's tab space you're trying to save). It can put all the tabs you're not using right now into one tab -- saving huge amounts of tab bar space.
To download and install a Chrome extension, all you need is the Opera extension, called "Download Chrome Extension."
When the page is minimized in Opera 12 and I close all the other tabs, the minimized page is not getting active, and that is exactly what I want to achieve. It is not a matter of saving space or something like that, I just want to have a tab which will not be focused after closing other tabs. Thanks for your answer.
Since pages always take up the whole application window in Chromium, there is neither Minimize nor Restore. No extension could truly minimize a page, but perhaps an extension could fake it by creating a new tab when the last non-minimized tab is closed. (You'll never get the empty background of 12, hence a new tab is the best they could do.)