Database Administrators can see below message on some systems
ORA-0xxxx: Message xxxx not found; No message file for product=RDBMS, facility=ORA
Usually it just indicates that the environment was wrongly setup.

To resolve the problem we just need to correctly setup ORACLE_HOME environment variable. We can do it by using export command

export  ORACLE_HOME=/oracle/app/oracle/product/11.1.0/db_1

To prove that the setting worked, we can use printenv command

printenv | grep ORACLE_HOME

ORACLE_HOME=/oracle/app/oracle/product/11.1.0/db_1

The best place to set ORACLE_HOME environment variable is to set it in profile file (the profile file location and name varies between operating systems and shells used the two most popular will be ~/.profile and ~/.bashrc).
Example part of .bashrc file

if [ -f /etc/bash_completion ]; then

   . /etc/bash_completion

fi

PATH="/oracle/app/oracle/product/11.1.0/db_1/bin/:$PATH"

ORACLE_HOME="/oracle/app/oracle/product/11.1.0/db_1"

ORACLE_SID=orcl

Example part of .profile file

ORACLE_HOME="/oracle/app/oracle/product/11.1.0/db_1"

ORACLE_SID=orcl

export ORACLE_HOME

export ORACLE_SID

On Windows systems ORACLE_HOME is set in registry, to edit it we need to run regedt32 and go to HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\ then we need to search for ORACLE_HOME key. (the location depends on Oracle version installed)