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 language codes (examples: http://www.lingoes.net/en/translator/langcode.htm)

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 swedish and not default browser:

launcher.exe /install /silent /allusers=yes /setdefaultbrowser=no /language=sv-se