[oracle@localhost /]$ export ORACLE_SID=XXX
[oracle@localhost /]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.1.0 Production on Fri Oct 11 14:38:45 2013
Copyright (c) 1982, 2009, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Release 11.2.0.1.0 - Production
SQL> create rollback segment users;
Rollback segment created.
SQL> alter rollback segment users online;
Rollback segment altered.