create table test (
    test_id number( 9 )
  , constraint pk_test primary key(test_id)
)
/
create table test2 (
    test_id number( 9 )
  , constraint pk_test2 primary key(test_id)
)
/
alter table test add constraint fk_test_test2
foreign key (test_id) references test2
deferrable initially deferred
/
alter table test2 add constraint fk_test2_test
foreign key (test_id) references test
deferrable initially deferred
/
insert into test values ( 1 );
insert into test2 values ( 1 );
commit;

set constraint fk_test_test2 immediate;

insert into test values ( 2 );

ORA- 02291 : integrity constraint (DAN.FK_TEST_TEST2) violated - parent key not found

set constraint fk_test_test2 deferred;
insert into test values ( 2 );
insert into test2 values ( 2 );
commit;

set constraints all deferred;
set constraints all immediate;