Randon House treats back order (two words) as the noun and back-order (hyphenated) as the verb, but I see on the net that other dictionaries vary and that all permutations are in use rather indiscriminately. If you wish to be conservatively correct or initiate a one-man effort to standardize the word forms, I suggest using the Random House templates. I shall try to do the same.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.