ossiculum vs os

ossiculum and os both are nouns.

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

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

Other hypernyms of ossiculum include bone.

ossiculum (noun)	os (noun)
a small bone; especially one in the middle ear	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening