When I installed Opera 18, my passwords were copied from my existing installation.
I installed it using the USB install option, then launched it using a command line to copy my Opera 11.64 profile, which also uses a USB install. Edit the command line below to match your paths, press the Windows key + R to open the Run dialogue, and paste the command. This is done after installing Opera 18, and launches it, importing bookmarks, wand.dat, and your current session. It also creates and HTML file of your notes on the desktop."<Full path to Opera 18>\launcher.exe" --presto-small-prefs-dir="<Full path to old Opera>\profile"
A standard install should copy your Opera profile from 12.16 AFAIK.