helmet vs ocular helmet

helmet and ocular helmet both are nouns.

Word	Noun	Adjective	Verb	Adverb
helmet	Yes	No	No	No
ocular helmet	Yes	No	No	No

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

helmet: armor plate that protects the head
ocular helmet: a leather or steel helmet similar to a nasal helmet but with a convex nasal and frontal head stripe that forms an ocular, popular in medieval Scandinavia.

Other hyponyms of helmet include armet, basinet, casque, heaume, cabasset, morion, pickelhaube, salade, sallet.

helmet (noun)	ocular helmet (noun)
a protective headgear made of hard material to resist blows	a leather or steel helmet similar to a nasal helmet but with a convex nasal and frontal head stripe that forms an ocular, popular in medieval Scandinavia.
armor plate that protects the head