In Fusion Middelware, an Oracle Instance is a container for all of system (non-JEE) components and processes.
An Oracle instance directory contains updatable files, such as configuration files, log files, and temporary files.
An Oracle instance is a peer of an WebLogic Server domain.
Both contain specific configurations outside of their Oracle homes.
Example of ORACLE_INSTANCE
It can reside anywhere; it need not be within the Middleware home directory.
$ORACLE_INSTANCE/bin/opmnctl registerinstance -adminHost $HOSTNAME -adminPort $ADMINPORT