Opera confirmed that from version 11 of Opera browser it will support extensions.Just like the Chrome and Firefox Opera too will have extensions.
What are Opera Extensions?
Extensions in Opera is a way for you to easily add new functionality to your Opera browser experience. Developers can easily create extensions using open standards (HTML5, CSS, JavaScript) and supported APIs.
Extensions will be based on the W3C Widget specifications and this is being considered for an Open Standard effort.
Will Opera Extensions be similar to extensions in other browsers?
Yes, Opera Extensions will be similar in some ways and we have tried to make it easy to port extensions from certain browsers.
We are committed to open web standards and this is an important part of our strategy, so if you find us deviating from other solutions out there, this might be why.
What does an extension look like?
Some extensions have user interface elements, but not all. An extension can for example add a button next to the Google Search field with a popup menu when you click on it. Other extensions don't have user interfaces at all, but run in the background.
How will I install Opera Extensions?
Once you have Opera 11 installed you can click on an extension and a small Install-window will appear. Click 'Install' and you're done. You can also drag and drop an extension onto Opera Desktop if you have an extension stored on your computer.
Read more here.