breastbone vs os

breastbone and os both are nouns.

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

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

breastbone: 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 breastbone include bone.

breastbone (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