The term OS hardening itself is misleading/vague to me as i am reading to understand it online here. It seems to me that its something related to making linux/unix box more secure.
To quote wikipedia:
There are usually guides by the OS vendor. Look for example at the Debian guide: http://www.debian.org/doc/manuals/securing-debian-howto/
Hardening is a process of securely configuring weak(vulnerability) point of a system like there may be unused port, services or useless software running that may create weak point in your system. these weak point may be used by other's to enter in your system.
For example in Linux systems there is a file called /etc/hosts is used to allow/disallow users as per policy. So you may need to change the permissions of this file chmod 700 /etc/hosts.allow or chmod 000 /etc/hosts.allow
For OS(Linux, Unix) hardening go to http://www.itsecurity.com/features/ubuntu-secure-install-resource/ http://www.security.berkeley.edu
You can use tool like PSysHard Hardening Framew0rk