Unpack the Opera_[VERSION]_Setup.exe with for example 7zip or WinRAR.
From the directory contaning the extracted files run:
launcher.exe /install /silent [SWITCHES]
The following switches can be used:
/allusers=yes /launchopera=no /desktopshortcut=no /setdefaultbrowser=no /pintotaskbar=no /startmenushortcut=no /installfolder="c:\Opera Browser" /language=en-us
Language is ISO language code.
If allusers=no the application is installed only for the current user and will not show up in add-remove programs for other users.
For example, silent install for all users in ukrainian and not default browser:
launcher.exe /install /silent /allusers=yes /setdefaultbrowser=no /language=uk-ua