This is another way of installing Opera extension which can be helpful when you are unable to install extension directly from Opera extensions site.
Manually Installing Opera extensions using OEX file
1.Visit Opera extensions site, right click on “Install” link of add-on
2. Click “Save Linked Content as” and click Save to your Computer with default extension file name.
3.Now drag and drop Opera extension file to Opera address bar, Opera asks to install the extension, install it.