ORA-01940 cannot drop a user that is currently connected
(HATASI İÇİN OTURUMU KILL ETMEK LAZIM)

SQL>conn sys as sysdba

SQL>GRANT alter system TO orhan;

SQL>select sid, serial#, username from v$session;

SQL>ALTER SYSTEM KILL SESSION ‘122,25574’ IMMEDIATE;

SQL> drop user orhan cascade;