Oracle 18c

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Create type a1 is object (
  name_a1 varchar2(10)
) NOT FINAL NOT INSTANTIABLE;
/
Create type b1 is object (
  name_b1 varchar2(10)
) NOT FINAL NOT INSTANTIABLE;
/
create type t_a1 is table of a1;
/
create type t_b1 is table of b1;
/

Create type b22 UNDER b1 (
  o_a t_a1
) INSTANTIABLE;
/
Create type a22 UNDER a1 (
  o_b t_b1
) INSTANTIABLE;
/
declare
  i a22;
  i1 VARCHAR(10);
  v_tb1 t_b1;
  v_b1 b1;
  v_ta1 t_a1;
  v_a1 a1;
begin
  i := a22('a22', t_b1(
                       b22('b22', t_a1(a22('a22_2', t_b1(b22('b22_2', t_a1())))))
                       )


           );

 
   dbms_output.put_line('i.name_a1='||i.name_a1);
   dbms_output.put_line('i.t_b1(1).name_b1='||i.o_b(1).name_b1);
   dbms_output.put_line('i.t_b1(1).name_b1='||treat(i.o_b(1) as b22).name_b1);
   dbms_output.put_line('i.o_b(1).o_a(1).name_a1='||treat(i.o_b(1) as b22).o_a(1).name_a1);
 
end;
/
1 rows affected

dbms_output:
i.name_a1=a22
i.t_b1(1).name_b1=b22
i.t_b1(1).name_b1=b22
i.o_b(1).o_a(1).name_a1=a22_2