long bone vs os

long bone and os both are nouns.

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

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

long bone: in limbs of vertebrate animals: a long cylindrical bone that contains marrow
os: rigid connective tissue that makes up the skeleton of vertebrates

Other hypernyms of long bone include bone.

long bone (noun)	os (noun)
in limbs of vertebrate animals: a long cylindrical bone that contains marrow	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening