braincase vs os

braincase and os both are nouns.

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

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

Other hypernyms of braincase include bone.

braincase (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