One of the positive side effects of switching to Blink, the same browser engine that Google Chrome uses, is that Opera users can tap right into the vast extension repository of that browser. That's at least the theory, but when you visit the Chrome web store you will notice that there is no way to install extensions as the add to Chrome button is not displayed on the page.
If you continue to research the issue, you will notice that you can install Chrome extensions from third party sources, download sites for example, without issues. The only factor preventing the installation on Chrome's Web Store is the removed add button there.
While I'm no fan of the layout of Google's web store, it is fair to say that it offers the safest way of installing Chrome extensions.
So how can we install Chrome extensions in Opera?
You need to download a new extension that was created for the purpose. Download Chrome Extension adds the download button back to the Chrome Web Store so that you can download and install extensions listed here.
Note: Most Chrome extensions seem to work just fine in Opera. There are however some that may not be compatible. I have tested several, Adblock Plus, Turn off the Lights or Stylish all seem to work just fine. Extensions that manipulate pages or features customized by Opera or Chrome do not. This includes many tab page extensions for example.
A new icon is added to Opera when you install the Download Chrome Extension in the browser. You need to click on its icon whenever you want to install extensions from Chrome's Web Store.
Just visit the store page of the extension you want to install and hit the icon to install it. Opera blocks the installation by default but displays a small notification at the top.
Extension was disabled because it is from unknown source. Go to the Extensions Manager to enable it.
Just hit ok here and open opera://extensions in the browser. Here you find the extension listed already but disabled. This is different from how Google Chrome handles these installations as it does not display third party installations at all but downloads them only to your local system.
To install the extension hit the button. You see a list of permissions that the extension requires to function.
Click install again to add it to the browser. You can alternatively click on the x-icon to remove it from Opera again.
Shared extensions between Chrome and Opera is definitely positive as Opera users can tap right into the vast Chrome extensions library right away and use most of the browser extensions from it in their browser.
Update: The extension has only been posted to Opera's internal Add-ons test server which is now blocking all attempts to access it from a remote location. It is therefore not available anymore right now.
Update 2: It has been officially released.