Try this:
Go to xbox system settings,,, go to storage,,, find Call of Duty Ghosts Update and Call of Duty Ghosts Onslaught,,, delete them both. Empty your cache (from memory in Storage there is a "Press Y to clear cache" at the bottom of page.
To be safe - go to your download history - make sure that there are no incomplete downloads.
Power cycle your xbox - pull cord out of wall for a couple of mnutes.
Go back to Ghosts - it will say - Update is needed - download.
Then go to store and re-download Onslaught.
If you can - make sure that you are using a wired connection while downloading.