ischium vs os

ischium and os both are nouns.

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

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

Other hypernyms of ischium include bone.

ischium (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