Usage: OPER [<nick> <password>]
This command will grant you status as a network operator for the server you are on. This gives you access to all the operator commands on that server.
If no parameter is given, then it will list the operators on the server you are on. This may be useful if you have an important problem or need assistance with abusive users.
/oper mike 9U7gh3
Despite having an oper definition configured into the server, you will not appear to be an oper or have access to any of the oper commands until you actually oper up. This command puts you on-duty and marks you as an oper. To de-oper you can use /UMODE -o.
In order to oper you must have an oper definition, your mask must match, you must put in the nick that is specified in your oper definiiton, and you must put in the correct case sensitive password. These are required to make sure that only the correct users can oper.