I'm not familiar with a /who command, and I can't find it in the mIRC help file either. There is /whois & /uwho, both of which are explained in your mIRC help file.
/who 0 o is for ircops depending on your ircd and services, it is usually used for ircops to list other ircops if the services breaks down. try /ns listops or /msg nickserv listops if your ircd setup supports it.