skull vs os

skull and os both are nouns.

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

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

Other hypernyms of skull include bone.

skull (noun)	os (noun)
the bony skeleton of the head of vertebrates	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening