oculus vs ocellus

oculus and ocellus both are nouns.

Word	Noun	Adjective	Verb	Adverb
oculus	Yes	No	No	No
ocellus	Yes	No	No	No

As nouns, ocellus is a hyponym of oculus; that is, ocellus is a word with a more specific, narrower meaning than oculus:

Other hyponyms of oculus include naked eye, peeper, oculus dexter, OD, oculus sinister, OS, simple eye, stemma, compound eye.

oculus (noun)	ocellus (noun)
the organ of sight	an eyelike marking (as on the wings of some butterflies); usually a spot of color inside a ring of another color
	an eye having a single lens