Create orapassword file

in init.ora file add 

remote_login_passwordfile EXCLUSIVE/SHARE

EXCLUSIVE - file is available only to one database and you can have more than one user
SHARE - file is available for multiple databases, only SYS user can be in this file.

For LINUX system file name has to be orapw (11g)

remote_login_passwordfile EXCLUSIVE/SHARE
SQL> show parameter password;
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
remote_login_passwordfile            string      EXCLUSIVE

SQL> select * from v$pwfile_users;
USERNAME                       SYSDB SYSOP SYSAS
------------------------------ ----- ----- -----
SYS                            TRUE  TRUE  FALSE

now I can login to DB using

sqlplus sys/PASS@SID as sysdba