Find Oracle Maintained (internal) users

 SYS @ GGATE1 >select username from dba_users where ORACLE_MAINTAINED ='Y' order by username;

USERNAME
--------------------------------------------------
ANONYMOUS
APPQOSSYS
AUDSYS
CTXSYS
DBSFWUSER
DBSNMP
DIP
GGSYS
GSMADMIN_INTERNAL
GSMCATUSER
GSMUSER
MDSYS
OJVMSYS
ORACLE_OCM
ORDDATA
ORDPLUGINS
ORDSYS
OUTLN
REMOTE_SCHEDULER_AGENT
SI_INFORMTN_SCHEMA
SYS
SYS$UMF
SYSBACKUP
SYSDG
SYSKM
SYSRAC
SYSTEM
MSYS
XDB
XS$NULL

30 rows selected.