That is because USCIS does not have an office in Oxnard. As the title of your question makes clear, the question refers to either the consulate where the immigrant visa was issued, or the USCIS office where the Adjustment of Status was granted. It is not the city you lived at.
Of course, the USCIS records will reflect the office where you adjusted status. If you are uncertain, you can schedule an InfoPass appointment at www.uscis.gov for information and assistance with the form. Normally I don't recommend going to USCIS on one's own, but if there are no problems or concerns with your status, it will likely be ok on an I-90 inquiry. If you have concerns, consult an attorney.
Scott D. Pollock