As I thought the country code for Oman is A40 NOT A4o. It would seem you need to change all Oman registrations!!

see: Chapter 4. Aircraft Nationality Marks, National Emblems, and Common Marks https://www.faa.gov/documentLibrary/media/Order/7340.2K_Bsc_w_Chg_1_2_and_3_dtd_8-12-21.pdf

Hi,

we had this debate a few years ago and it was settled decisively by document published by the CAA of Oman which clearly stated that Omani registrations are letter A, number 4 the the letter O. The document is here, see page 33: https://www.caa.gov.om/upload/files/CAR-47%20Aircraft%20Registration%20%26%20Marks%20v0.18%20JNC%20-%20FINAL%20REVISION%20-%20081120%281%29.pdf

The document you mentioned it does indeed look like a number 0 not a letter O which is intriguing. However, I would venture that in this case the Omani CAA opinion is probably correct and the US FAA are wrong. :)