This command, when used successfully, grants IRC operator privileges to the issuer. For this command to work, the user in question must be running the command from an approved host on a server that has their oline information. Global IRC operators and above will have oline information on every server. Once the operator is recognized, they are given user mode +o and are automatically joined to any channels critical to the performance of their duties.
This command sets user mode +o. This is one of only two user modes that users cannot set directly, the other being +r. Unlike user mode +r however, user mode +o can be unset by the user at any time. The handling of this command is client specific. Consult your client's documentation if you have any doubt as to how to use this command.
In most clients, this command will accept 2 arguments, the operator block name that you are identifying too, and the password used to identify to that block. Some clients require that this command be issued without any arguments, to prevent accidentally sharing your operator password with channels or users. If your client requires that you issue your password as one of the arguments, then you should issue this command in your server buffer so that an accidental release of your operator password to other users cannot happen.