os ischii vs os

os ischii and os both are nouns.

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

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

os ischii: one of the three sections of the hipbone; situated below the ilium
os: rigid connective tissue that makes up the skeleton of vertebrates

Other hypernyms of os ischii include bone.

os ischii (noun)	os (noun)
one of the three sections of the hipbone; situated below the ilium	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening