For what I understood, there's one condition missing in the previous solutions.
Here's my guess (since I have nothing to test on)
Declare
@OrdNbr CoNumType
Select ISNULL( order_nbr, 'xxxxxx') order_nbr, field1, field2, field3
from table
where order_nbr = @Ord_nbr
OR( order_nbr IS NULL AND @Ord_nbr IS NULL)