A listing on the Australian Xbox Store seems to indicate that the original Dark Souls will become a Xbox One backwards compatible title. This can be viewed here, but note you must change your store location to AU.
Furthermore, the listing states that anyone who digitally pre-orders the upcoming Dark Souls 3 will get the original game for free. Players will get the original title within 7-10 days after pre-ordering.
Finally, though not confirmed, the original Dark Souls could be backwards compatible as of next week. Bandai Namco has yet to comment on this listing or its validity.