Opacityutil is a command line utility that can perform some of the more basic operations of Opacity. It is included within Opacity's application bundle. You can install opacityutil from the Goodies Preferences to link from /usr/bin/opacityutil to the binary in the application bundle to allow you to access opacityutil more easily.
Opacityutil can convert files between any of Opacity's supported formats (including loading and setting icons for files), and can also build all of the factories from Opacity images. It can also copy an image to the clipboard. To see all of the options and how to use opacityutil, enter "opacityutil -help" in Terminal (after installing opacityutil from the Goodies Preferences).