os pubis vs os

os pubis and os both are nouns.

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

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

os pubis: one of the three sections of the hipbone; together these two bones form the front of the pelvis
os: rigid connective tissue that makes up the skeleton of vertebrates

Other hypernyms of os pubis include bone.

os pubis (noun)	os (noun)
one of the three sections of the hipbone; together these two bones form the front of the pelvis	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening