In my case the Oath letter came from the DO, and there is some logic for it:
1/ once my file has been the DO for the interview, it stays at the DO
2/ the DO having the files of interviewed applicants helps contact them should an Oath Ceremony be postponed for any reason.
It makes sense that the DO may have had less active personnel on duty between 12/23 and now, but I would think their activities should be resuming at normal pace in the next few days.
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Detroit DO - NSC
09/28/98 Entered on K-1 (Fiance) Visa
12/19/98 Married with US Citizen (still happily married 11 years after )
12/24/98 Filed I-485, I-130, I-765 in OKC (NSC)
01/03/99 Moved to Texas (TSC)
05/xx/00 Moved to Eastern PA (VSC)
11/xx/00 Moved to Central NJ (VSC)
03/19/01 LPR
03/xx/03 Received GC dated 03/19/2001
03/20/07 N-400 Priority Date
04/12/07 FP done
09/19/07 Interview passed

10/26/07 Naturalized Citizen
