sternum vs os

sternum and os both are nouns.

Word	Noun	Adjective	Verb	Adverb
sternum	Yes	No	No	No
os	Yes	No	No	No

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

sternum: the flat bone that articulates with the clavicles and the first seven pairs of ribs
os: rigid connective tissue that makes up the skeleton of vertebrates

Other hypernyms of sternum include bone.

sternum (noun)	os (noun)
the flat bone that articulates with the clavicles and the first seven pairs of ribs	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening