Usage: OP <#channel> [<nick>]
This will op yourself or a user in a channel, but the mode change is done by the server, you need to have AOP or higher access to the channel to use this command. If no nick is specified, it will op yourself.
It's particularly useful if you are not opped but need to quickly op someone else or yourself.
/op #mychannel Zorgor
op #mychannel
See also CMODE +o and VOICE.
This replaces the service command /CS OP.