national capital vs capital of Oman

national capital and capital of Oman both are nouns.

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

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