How to Query Oracle Hidden Parameters

The hidden parameters cannot queried from v$parameter or "show parameter" unless they are changed explicitly by "alter system" command or in init.ora. However having SYSDBA role make you able to query those parameters alongside their values and descriptions as follow:

  SELECT a.ksppinm Param,
         b.ksppstvl SessionVal,
         c.ksppstvl InstanceVal,
         a.ksppdesc Descr
    FROM x$ksppi a, x$ksppcv b, x$ksppsv c
   WHERE     a.indx = b.indx
         AND a.indx = c.indx
         AND a.ksppinm LIKE '/_%' ESCAPE '/'
ORDER BY 1

Caution:

Hidden and undocumented parameters are very critical and only be useful to Sr. DBA who clearly understand how this parameter would change the internal behaviour for Oracle. So don't change any of them unless suggested by Oracle Support for specific case.