Search notes:

Component (directories) under $ORACLE_HOME

Some of the directories found under $ORACLE_HOME correspond to «components» (see $ORACLE_HOME/lib/facility.lis).

mesg subdirectories

Each(?) component maintains a mesg subdirectory which in turn store message files for facilities. These message files have the suffixes .msg if they're readable or .msb if they're compiled into a binary form.
English readable message files can be found with 
$ find $ORACLE_HOME -name '*us.msg' | sort
The source code of ORACLE_HOME/bin/oerr.pl is quite helpful when trying to understand components, facilities and message files.
See also MOS Note 985974.1

See also

Index