ON (t1.a = t2.a and t1.b = t2.b and ... t1.z = t2.z)

NVL(t1.a, -1) == NVL(t2.a, -1)

on a.x is null and b.x is null or a.x = b.x

create table t1(a int, b int, s varchar2(20));
create table t2(a int, b int, s varchar2(20));

insert into t1 values(1, 1, '1');
insert into t1 values(2, 2, '2');
insert into t1 values(3, 3, '3');
insert into t1 values(NULL, 1, 'NULL.a.1');
insert into t1 values(NULL, 2, 'NULL.a.2');
insert into t1 values(NULL, 3, 'NULL.a.3');
insert into t1 values(NULL, NULL, 'NULL.both.1');
insert into t1 values(NULL, NULL, 'NULL.both.2');

insert into t2 values(1, 1, '1');
insert into t2 values(2, 2, '2');
insert into t2 values(NULL, 1, 'NULL.a.1');
insert into t2 values(NULL, 2, 'NULL.a.2');
insert into t2 values(NULL, NULL, 'NULL.both.1');
insert into t2 values(NULL, NULL, 'NULL.both.2');

SQL> select * from t1 join t2 on NVL(t1.a, -1) = NVL(t2.a, -1) and NVL(t1.b, -1) = NVL(t2.b, -1);

         A          B S                             A          B S
---------- ---------- -------------------- ---------- ---------- --------------------
         1          1 1                             1          1 1
         2          2 2                             2          2 2
                    1 NULL.a.1                                 1 NULL.a.1
                    2 NULL.a.2                                 2 NULL.a.2
                      NULL.both.1                                NULL.both.2
                      NULL.both.1                                NULL.both.1
                      NULL.both.2                                NULL.both.2
                      NULL.both.2                                NULL.both.1

8 rows selected.