Opera allows users to add custom user JavaScript that can add additional functionality to the browser, however the User JavaScript cannot run out of the box, and you will require to tune some settings in order to get it working.

Here is a step by step tutorial that will teach you how to install user JavaScript in Opera.

Step 1: Go to Tools > Preferences or press Ctrl + F12 shortcut key.

Step 2: Navigate to Content in the left hand side menu and click on the JavaScript Options button.

Step 3: In the new options window click on the Choose button next to User JavaScript files and select a folder where you will be storing the JS files. Remember, this folder can be anything and you must store all the downloaded Opera User JS files in this folder for it to work.

To install new User JavaScript, just copy the file to the User JavaScript files directory, to remove the script delete it from the folder.