Apparently you can unzip the installer with 7zip.
Here is a quote from the opera desktop team blog.
We are still left with one third-party tool, 7-zip, which we need to
bundle the opera package in a single self-extractable file, which is
the file you download. When it is run, it simply unpacks everything
in a temporary folder and starts up opera in installer mode. For
those of you with 7-zip installed, you can actually just extract
opera wherever you want and run it.