cranium vs os

cranium and os both are nouns.

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

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

Other hypernyms of cranium include bone.

cranium (noun)	os (noun)
the part of the skull that encloses the brain	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening