There is only one file type that uses the .KO extension:
Linux Kernel Module File
Module file used by the Linux kernel, the central component of the Linux operating system; contains program code that extends the functionality of the Linux kernel, such as code for a computer device driver; can be loaded without restarting the operating system; may have other required module dependencies that must be loaded first.
KO modules may be loaded by using the insmod Linux program. Installed kernel modules can be listed using the lsmod program, or they may be browsed in the /proc/modules directory.
As of Linux kernel version 2.6, KO files are used in place of .O files and contain additional information that the kernel uses to load modules. The Linux program modpost can be used to convert O files into KO files.
NOTE: KO files may also be loaded by FreeBSD using the kldload program.
Updated: July 3, 2010