You can try those conditions :

SELECT *
FROM mytable
WHERE (
      columnA <> columnB 
      AND NOT (columnA = 'bb' and columnB = 'oo' 
               or columnA = 'oo' and columnB = 'bb')
      ) OR columnA IS NULL OR columnB IS NULL;

Which, for the sample data:

create table mytable (
  ColumnA   varchar(20),
  ColumnB   varchar(20)
);
insert into mytable values
('ABC', '1'),
('oo',  'bb'),
('oo',  NULL),
(NULL,  'bb'),
(NULL,  NULL),
('BCD', '2'),
('bb',  'oo');

Outputs :

ColumnA ColumnB
ABC     1
oo      null
null    bb
null    null
BCD     2

Demo here

You can use the condition to exclude the records and then negate them with NOT as follows:

SELECT *
FROM   YOUR_TABLE
WHERE  COLUMN_A <> COLUMN_B
AND    NOT ((COLUMN_A = 'oo' AND COLUMN_B = 'bb') 
             OR (COLUMN_A = 'bb' AND COLUMN_B = 'oo'))