Query to get ORACLE_HOME path in Oracle 11G
There is  sys_context function

that works for Oracle 12c


select SYS_CONTEXT ('USERENV','ORACLE_HOME') from dual;


This does not work for lower versions of Oracle.
For 11g and lower versions, I don't know a simple method. 

PS: I read @Balasz' answer and now I know a simple answer for version 11g.