oracle@ywdb ~]$ sqlplus "/as sysdba"

SQL*Plus: Release 11.1.0.7.0 - Production on Tue Apr 29 15:40:38 2014

Copyright (c) 1982, 2008, Oracle. All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.1.0.7.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> alter user sys identified by oracle
2 ;

User altered.

SQL> alter user sys identified by oracle;

User altered.

SQL> alter user sys identified by manager;

User altered.

SQL> grant sysdba to system;
grant sysdba to system
*
ERROR at line 1:
ORA-01994: GRANT failed: password file missing or disabled

SQL> exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.1.0.7.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
You have new mail in /var/spool/mail/oracle
[oracle@ywdb ~]$ orapwd file=$ORACLE_HOME/dbs/orapw$ORACLE_SID password='manager' entries=10
[oracle@ywdb ~]$ sqlplus "/as sysdba"

SQL*Plus: Release 11.1.0.7.0 - Production on Tue Apr 29 15:49:03 2014

Copyright (c) 1982, 2008, Oracle. All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.1.0.7.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> grant sysdba to system;

Grant succeeded.

SQL>

system用户没有dba权限的。

但是想授予他权限的话，

那么必须添加一个密码文件。

然后才能授予。