SQL> create or replace
2 type person_t as object
3 ( first_name varchar2(30)
4 , last_name varchar2(30)
5 , birthdate date
6 )
7 /

Type is aangemaakt.

SQL> create or replace
2 type personal_relation_t as object
3 ( relationship_type varchar2(20)
4 , start_date date
5 , from_person person_t
6 , with_person person_t
7 )
8 /

Type is aangemaakt.

SQL> DROP TYPE person_t FORCE
2 /

Type is verwijderd.

SQL> create or replace
2 type person_t as object
3 ( first_name varchar2(30)
4 , last_name varchar2(30)
5 , birthdate date
6 , gender varchar2(1)
7 )
8 /

Type is aangemaakt.

SQL> DESC person_t
Naam Null? Type
—————————————– ——– —————————-
FIRST_NAME VARCHAR2(30)
LAST_NAME VARCHAR2(30)
BIRTHDATE DATE
GENDER VARCHAR2(1)

SQL> DESC personal_relation_t
Naam Null? Type
—————————————– ——– —————————-
RELATIONSHIP_TYPE VARCHAR2(20)
START_DATE DATE
FROM_PERSON PERSON_T
WITH_PERSON PERSON_T

That’s not true. It can be altered by issuing ALTER TYPE statement (at least from 9i release 1. I don’t have older doc).
http://download.oracle.com/docs/cd/A91202_01/901_doc/server.901/a90125/statements_43.htm#2057830

tested in 11.1.0.6.0:

SQL> create type a as object(a number);
2 /

Type created

SQL> create type b as object(b a);
2 /

Type created

SQL> alter type a add attribute (c number) invalidate;

Type altered

SQL> alter type a add attribute (d number) cascade;

Type altered

SQL> desc a
Element Type
——- ——
A NUMBER
C NUMBER
D NUMBER

SQL> desc b
Element Type
——- —-
B A