ramus vs os

ramus and os both are nouns.

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

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

Other hypernyms of ramus include bone.

ramus (noun)	os (noun)
the posterior part of the mandible that is more or less vertical	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening