select dbms_metadata.get_ddl('TABLE','Table-name','Schema') from dual;

SET serveroutput ON format wraped; 
Declare
Ergebnis varchar2(9000);
BEGIN
  FOR Tabelle IN 
      (SELECT table_name FROM all_tables WHERE owner='schema') 
  LOOP
      dbms_output.put_line(Tabelle.table_name);
      select dbms_metadata.get_ddl('TABLE',Tabelle.table_name,'schema') into Ergebnis from dual;
      dbms_output.put_line(Ergebnis);
  END LOOP;
END;

select dbms_metadata.get_ddl('TABLE',table_name,owner) 
from dba_tables
/

SELECT DBMS_METADATA.get_ddl ('TABLE', table_name)
  FROM user_all_tables
 WHERE nested = 'NO';