round bone vs os

round bone and os both are nouns.

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

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

Other hypernyms of round bone include bone.

round bone (noun)	os (noun)
bones that are round in shape	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening