why OEM allow to connect system as normal and sys dont? In OEM I can connect with system as normal but with sys cant connect as normal. Why is that?
Because this is how it was designed.

7.3.2 SYSDBA and SYSOPER System Privileges


  The SYS user is automatically granted the SYSDBA privilege upon
  installation. When you log in as user SYS, you must connect to the
  database as SYSDBA or SYSOPER.