And, according to the streamer who was playing, the DLC is almost all on-disc, including the pre-order bonuses.
If you didn't purchase Omega, you head to his area during a part of the story (mandatory), and near the end... He says you should fight him, but you have to get the DLC first.
Which is impossible, if SE wasn't releasing as normal DLC.
EDIT: And people have already beaten it.
ENDING SPOILERS:
Lightning & Serah were brainwashed by Etro to believe were sisters, and Etro is evil. In the end, every major character dies except for Hope and Noel.
The game ends with a "To Be Continued..." screen.
Crappy ending, I'd say.