If you specify an Oracle objectname (including usernames) without quotes, Oracle searches for the name in upper case without special characters. It is advisable to not use mixed or lower case names or special characters in your names for Oracle objects for this reason. Oracle can accept lower/mixed case names and special characters, if you specify quotes around the names. See this example:
SQL> create user "first/dbgokul" identified by foo
2 /
Gebruiker is aangemaakt.
SQL> select username from dba_users where username like 'fi%'
2 /
USERNAME
------------------------------
first/dbgokul
1 rij is geselecteerd.
SQL> drop user first/dbgokul
2 /
drop user first/dbgokul
*
FOUT in regel 1:
.ORA-00921: Onverwacht einde van SQL-opdracht.
SQL> drop user "first/dbgokul"
2 /
Gebruiker is verwijderd.
Regards,
Rob.