The Opera web browser does not need any plugins or rxtensions in order to be able to install and run user Javascripts. From version 8 onwards support for user scripts has been built into the basic functioning of the Opera browser.
All that you need to do to be able to install user scripts for use with Opera is to tell Opera where to find them. To do this you simply go to the Advanced Preferences window by selecting Preferences from the Tools menu and then selecting the Advanced tab. You then select Content from the list on the left of the window and then the Javascript Options button. There is an input field at the bottom of the Javascript Options window called My Javascript Files. This input field identifies the folder on your computer where your user scripts are stored. Simply browse to or enter the location of the folder where you intend to store your user scripts. You can even create a new folder to hold the scripts if you like.
With that defined you now need only copy any user scripts that you want to use into that folder for Opera to immediately apply them to any subsequent pages loaded into Opera (even if Opera is already open). To apply a new user script to an already open page simply refresh the page.
Note that while you can use any extension you like for Javascripts that you put on your web site, Javascripts that are to run as user scripts must have a .js or .user.js extension in order to run. Those with a .user.js extension are assumed to be Greasemonkey user scripts which have additional information in the script that tells the browser which web sites that the script should run on.
