Connecting with a user which has SYSDBA privilege, you act like SYS user

bash-3.2$ sqlplus "/as sysdba"

SQL*Plus: Release 11.2.0.1.0 Production on Thu Jul 23 11:12:10 2015

Copyright (c) 1982, 2009, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>
SQL>
SQL> show user
USER is "SYS"
SQL>
SQL>
SQL>
SQL> show parameter REMOTE_LOGIN_PASSWORDFILE

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
remote_login_passwordfile            string      EXCLUSIVE
SQL>
SQL>

SQL>

SQL> select * from v$pwfile_users;

USERNAME                       SYSDB SYSOP SYSAS

------------------------------ ----- ----- -----

SYS                            TRUE  TRUE  FALSE

SQL>

SQL>

SQL>

SQL> create user admin identified by admin;

User created.

SQL> grant sysdba to admin;

Grant succeeded.

SQL> grant dba to admin;

Grant succeeded.

SQL>

SQL> select * from v$pwfile_users;

USERNAME                       SYSDB SYSOP SYSAS

------------------------------ ----- ----- -----

SYS                            TRUE  TRUE  FALSE

ADMIN                          TRUE  FALSE FALSE

SQL>

SQL>
SQL> grant sysoper to admin;

Grant succeeded.

SQL> select * from v$pwfile_users;

USERNAME                       SYSDB SYSOP SYSAS
------------------------------ ----- ----- -----
SYS                            TRUE  TRUE  FALSE
ADMIN                          TRUE  TRUE  FALSE


SQL>conn admin/admin
SQL> show user
USER is "ADMIN"
SQL>
SQL> select * from v$tablespace;

       TS# NAME                           INC BIG FLA ENC
---------- ------------------------------ --- --- --- ---
         0 SYSTEM                         YES NO  YES
         1 SYSAUX                         YES NO  YES
         4 USERS                          YES NO  YES
         3 TEMP                           NO  NO  YES
         6 EXAMPLE                        YES NO  YES
         5 UNDOTBS2                       YES NO  YES

6 rows selected.

but when you go to shutdown  databasethen use sysdba role

SQL>

SQL> conn admin/admin as sysdba

Connected.

SQL>

SQL> shut immediate;

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL>

SQL> conn admin/admin as sysdba
Connected.
SQL> show user
USER is "SYS"
SQL>
SQL>

if use without  as sysdba so you got ora error

SQL>

SQL> conn admin/admin

Connected.

SQL>

SQL> shut immediate

ORA-01031: insufficient privileges

SQL>

OS Authentication on Oracle Server in 11gr2

Connect as sysdba

should be oracle user and os user same
in my case oracle is os user

SQL>
SQL>
SQL>
SQL> create user "OPS$ORACLE" identified by externally;

User created.

SQL> grant connect,resource to "OPS$ORACLE";

Grant succeeded.

SQL> show parameter auth

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
ldap_directory_sysauth               string      no
os_authent_prefix                    string      ops$
remote_os_authent                    boolean     FALSE
SQL>
SQL> exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
bash-3.2$
bash-3.2$
bash-3.2$ sqlplus /

SQL*Plus: Release 11.2.0.1.0 Production on Wed Jul 22 12:06:26 2015

Copyright (c) 1982, 2009, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> show user
USER is "OPS$ORACLE"
SQL>

Oracle Error: ORA-00600: internal error code, arguments: [kcratr_nab_less_than_odr], [1], [4], [18313], [18410], [], [], [], [], [], [], []

oracle 11gr2,solaris10

After Power Fail Alter database open fails with

ORA-00600: internal error code, arguments: [kcratr_nab_less_than_odr]


======================================================================================

SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-00600: internal error code, arguments: [kcratr_nab_less_than_odr], [1],
[4], [18313], [18410], [], [], [], [], [], [], []


SQL> shut abort;
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area  647204864 bytes
Fixed Size                  2213816 bytes
Variable Size             423626824 bytes
Database Buffers          218103808 bytes
Redo Buffers                3260416 bytes
Database mounted.
SQL> recover database until cancel using backup controlfile;
ORA-00279: change 1046055 generated at 07/15/2015 11:46:31 needed for thread 1
ORA-00289: suggestion :
/opt/oracle/flash_recovery_area/TEST11G/archivelog/2015_07_15/o1_mf_1_4_%u_.arc
ORA-00280: change 1046055 for thread 1 is in sequence #4


Specify log: {<RET>=suggested | filename | AUTO | CANCEL}
auto
ORA-00308: cannot open archived log
'/opt/oracle/flash_recovery_area/TEST11G/archivelog/2015_07_15/o1_mf_1_4_%u_.arc
'
ORA-27037: unable to obtain file status
Solaris-AMD64 Error: 2: No such file or directory
Additional information: 3


ORA-00308: cannot open archived log
'/opt/oracle/flash_recovery_area/TEST11G/archivelog/2015_07_15/o1_mf_1_4_%u_.arc
'
ORA-27037: unable to obtain file status
Solaris-AMD64 Error: 2: No such file or directory
Additional information: 3


SQL> recover database until cancel using backup controlfile;
ORA-00279: change 1046055 generated at 07/15/2015 11:46:31 needed for thread 1
ORA-00289: suggestion :
/opt/oracle/flash_recovery_area/TEST11G/archivelog/2015_07_15/o1_mf_1_4_%u_.arc
ORA-00280: change 1046055 for thread 1 is in sequence #4


Specify log: {<RET>=suggested | filename | AUTO | CANCEL}
cancel
Media recovery cancelled.
SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-01589: must use RESETLOGS or NORESETLOGS option for database open


SQL> alter database open resetlogs;

Database altered.

SQL>
SQL> archive log list
Database log mode              No Archive Mode
Automatic archival             Disabled
Archive destination            USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence     1
Current log sequence           1
SQL>
SQL>