Why "Sultanate of Muscat and Oman" are within Oman. It should be listed in its own head (own dropdown).
Please see the following wiki link.
https://en.wikipedia.org/wiki/Muscat_and_Oman
For the following reasons I believe it should be counted as a separate country:
1) It was independent state under British Protectorate from (1892-1971).
2) It had differnt flag from current day Oman.
3) The name of the country was "Sultanate of Muscat and Oman".
4) It was a country that encompassed the present day Sultanate of Oman and parts of the United Arab Emirates.