Operaised -- The blog

Apart from the right and left click buttons on the mouse, the other buttons on your mouse are probably the most neglected buttons in Opera, maybe all they were designed to do is back and forward.

Not so fast there, my friend, because as you can edit those mouse buttons and their actions, and even so in parts of opera where the shortcut wouldn't normally apply.

First of all, to start editing or making your own mouse shortcuts, go to Tools > Preferences > Advanced > Shortcuts and click "Edit" button near selected mouse setup.

What you should see is a list of places where the shortcut can be used in.
-Application
-Browser Window
-Mail Window
-Document Window

And so on, until Advanced section at the bottom, with even more places to put in shortcuts.

Here is a key of the button numbers:

Button3 // Middle mouse button
Button6 // Special back button on some mice.
Button7 // Special forward button on some mice.
FlipBack // Its a special "rocker" gesture, where 2 buttons are hit simultaneously, but for this one, the right click is held before the left click.
FlipForward // Same as above, but this one, the left click is held before the right click.

And the list of known about sections.

Application // Section that overrides all the other sections! Only put shortcuts in here that you are guaranteed not to use anywhere else in Opera.
Browser Window // Section that only resides to the browsing window, so in theory should not activate in opera's email compose window for instance.
Mail Window // Section that applies to the mail interface, for example the mail listing window, or the email compose window.
Document Window // I think this section is roughly the same as Browser window, post in comments if there is any difference between the two.
Compose Window // This section should only apply to the email compose window.
Bookmarks/Contacts/History/Mail/Transfers/Windows/Notes panel // Sections that the shortcut is in would only apply to the retrospective sidebars.

Whats known in the advanced section:

Dialog // Section where the shortcuts would only apply to the dialogs, like the preferences dialog for example.
Form // I'm guessing this would work in html forms.
Widget Container // These are the toolbars and their contents. Not specific to a toolbar though, to all.
Browser/Bookmarks/Contacts/Notes Widget // sections inside the retrospective areas. 
Button Widget // For buttons, obviously, on the page I think.
Radiobutton/Checkbox/Dropdown/List/Tree Widget // Has the affect on html elements that are those maybe in Opera UI this has an affect too.
Edit Widget // Literally anywhere you can enter text.
Address Dropdown Widget // The address field specifically.

For example you could put a "Paste and Go | Paste" command with the "Button3" as button, and clicking that would either paste and go in address bar and search fields inside opera, or paste into a text field on the page.

Experiment and have fun!