ischial bone vs os

ischial bone and os both are nouns.

Word	Noun	Adjective	Verb	Adverb
ischial bone	Yes	No	No	No
os	Yes	No	No	No

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

ischial bone: 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 ischial bone include bone.

ischial bone (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