sesamoid bone vs os

sesamoid bone and os both are nouns.

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

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

sesamoid bone: any of several small round bones formed in a tendon where it passes over a joint
os: rigid connective tissue that makes up the skeleton of vertebrates

Other hypernyms of sesamoid bone include bone.

sesamoid bone (noun)	os (noun)
any of several small round bones formed in a tendon where it passes over a joint	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening