metacarpal bone vs os

metacarpal bone and os both are nouns.

Word	Noun	Adjective	Verb	Adverb
metacarpal bone	Yes	No	No	No
os	Yes	No	No	No

As nouns, os is a hypernym of metacarpal bone; that is, os is a word with a broader meaning than metacarpal bone:

Other hypernyms of metacarpal bone include bone.

metacarpal bone (noun)	os (noun)
any bone of the hand between the wrist and fingers	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening