How to drop all user tables?

SELECT 'DROP TABLE "' || TABLE_NAME || '" CASCADE CONSTRAINTS;' 
FROM user_tables;

user_tables is a system table which contains all the tables of the user the SELECT clause will generate a DROP statement for every table you can run the script

To remove all objects in oracle :

1) Dynamic

DECLARE
CURSOR IX IS
SELECT * FROM ALL_OBJECTS WHERE OBJECT_TYPE ='TABLE' 
AND OWNER='SCHEMA_NAME';
 CURSOR IY IS
 SELECT * FROM ALL_OBJECTS WHERE OBJECT_TYPE 
IN ('SEQUENCE',
'PROCEDURE',
'PACKAGE',
'FUNCTION',
'VIEW') AND  OWNER='SCHEMA_NAME';
 CURSOR IZ IS
 SELECT * FROM ALL_OBJECTS WHERE OBJECT_TYPE IN ('TYPE') AND  OWNER='SCHEMA_NAME';
BEGIN
 FOR X IN IX LOOP
   EXECUTE IMMEDIATE('DROP '||X.OBJECT_TYPE||' SCHEMA_NAME.'||X.OBJECT_NAME|| ' CASCADE CONSTRAINT');
 END LOOP;
 FOR Y IN IY LOOP
   EXECUTE IMMEDIATE('DROP '||Y.OBJECT_TYPE||' SCHEMA_NAME.'||Y.OBJECT_NAME);
 END LOOP;
 FOR Z IN IZ LOOP
   EXECUTE IMMEDIATE('DROP '||Z.OBJECT_TYPE||' SCHEMA_NAME.'||Z.OBJECT_NAME||' FORCE ');
 END LOOP;
END;
/

2)Static

    SELECT 'DROP TABLE "' || TABLE_NAME || '" CASCADE CONSTRAINTS;' FROM user_tables
        union ALL
        select 'drop '||object_type||' '|| object_name || ';' from user_objects 
        where object_type in ('VIEW','PACKAGE','SEQUENCE', 'PROCEDURE', 'FUNCTION')
        union ALL
        SELECT 'drop '
        ||object_type
        ||' '
        || object_name
        || ' force;'
        FROM user_objects
        WHERE object_type IN ('TYPE');

Please follow the below steps.

begin
  for i in (select 'drop table '||table_name||' cascade constraints' tb from user_tables) 
  loop
     execute immediate i.tb;
  end loop;
  commit;
end;
purge RECYCLEBIN;