File with a .KO extension contains source code of a module which expands the functionality of a Linux system kernel. These files, since the 2.6 version have substituted the .O files, due to the fact that they have additional information useful during loading modules through a kernel. O files can be converted into KO with the modpost tool.

KO modules loading (for instance the ones comprised of device’s drivers) can be performed without the necessity to restart a computer, with the insmod or kldload (in FreeBSD) tool, however, sometimes initially other modules on which a given file is dependant have to be loaded first. In order to check which modules are installed, user has to browse through /proc/modules directory or use the lsmod command.