Attacking Double DES (OFB Mode) given the initialization vector and not much else?
Read up on weak keys on Wikipedia.

The weak keys are the ones that result in the left and right key halves at the output of the Permuted Choice function being either all 1's or all 0's. So you can test all possibilities for weak keys to break the OFB mode.

What is more, if weak keys are used DES becomes idempotent, thus double encryption gives the plaintext back. This, however, would normally only work in ECB mode.