ilium vs os

ilium and os both are nouns.

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

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

Other hypernyms of ilium include bone.

ilium (noun)	os (noun)
the upper and widest of the three bones making up the hipbone	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening