You previously indicated that you had interview over 1 year ago and still haven't received oath letter. It may very well be that your oath letter was lost in the mail.
Do this:
Call USCIS at 1-800-375-5283, choose option 1-2-3, tell the agent it's been more than 30 days since you applied and haven't received receipt (don't tell them anything else, including you had interview). The agent will transfer your call to an IO. Tell the IO you haven't received oath letter. The IO will be able to tell if an oath has been scheduled for you. Alternatively, schedule an Infopass appointment to get the same info.
Btw, have you been physically present in the US after your interview?