oper [<nick> [<password>]]
The OPER command is used to gain irc operator status on the current server, with the stipulation that the specified (or current) nickname and password match an O: line entry in the server's configuration. If a password is not specified, the client will prompt for one.
Specifying the password on the input line is not recommended, for many reasons, the most obvious being that anyone with a line of sight to your monitor can see what your password is. When the client prompts for a password, it does not display it as it is typed. Further, the client's command history may be written to a file, which would compromise the password if the history file is world-readable.