Know ORACLE_HOME from Sqlplus

exec this as sysdba..

9i and above:
SELECT substr(file_spec,1,instr(file_spec,'lib')-2) ORACLE_HOME FROM dba_libraries WHERE library_name='DBMS_SUMADV_LIB';

10g and above:
var OHM varchar2(100);
EXEC dbms_system.get_env('ORACLE_HOME', :OHM) ;
PRINT OHM