Unable to mount oracle instance
Exit sqlplus.

Make sure your os user is part of the DBA group (ORA_DBA in Windows).

Make sure ORACLE_SID is set to the SID of the database and unset TWO_TASK:

Windows:

set ORACLE_SID=ORCL
set TWO_TASK=


or

Unix/Linux

ORACLE_SID=ORCL
TWO_TASK=
export TWO_TASK


Now, connect as sysdba:

sqlplus / as sysdba


Now, startup the database:

SQL&gt; startup mount;

SQL&gt; alter database open;