ODST was finally made available to download on the Xbox One. It’s still downloading but it looks like the game is being treated not as DLC for the MCC but just as a separate game. In which case I feel like maybe they should have made it the complete ODST experience and put Firefight in?

UPDATE: So it was actually DLC for MCC. The file downloaded separately as if it were just going to be its own game in my dashboard, but upon completion of the download, it just attached the ODST levels to the Halo 3 sections of the game. I still really  want Firefight in this game though.