Para listar los usuarios en oracle ejecutar:

 

SQL> select username from dba_users;

USERNAME
--------------------------------------------------------------------------------
SYSTEM
SYS
QUIZZAPPUSER
MGMT_VIEW
OUTLN
WKSYS
OLAPSYS
SI_INFORMTN_SCHEMA
FLOWS_030000
ORDPLUGINS
WKPROXY

USERNAME
--------------------------------------------------------------------------------
XDB
SYSMAN
ANONYMOUS
CTXSYS
WK_TEST
WMSYS
DBSNMP
EXFSYS
ORDSYS
MDSYS
FLOWS_FILES

USERNAME
--------------------------------------------------------------------------------
APEX_PUBLIC_USER
DIP
XS$NULL
TSMSYS
ORACLE_OCM

27 rows selected.