capital of Oman vs national capital

capital of Oman and national capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
capital of Oman	Yes	No	No	No
national capital	Yes	No	No	No

capital of Oman (noun)	national capital (noun)
a port on the Gulf of Oman and capital of the sultanate of Oman	the capital city of a nation