oregano vs herb

oregano and herb both are nouns.

Word	Noun	Adjective	Verb	Adverb
oregano	Yes	No	No	No
herb	Yes	No	No	No

As nouns, herb is a hypernym of oregano; that is, herb is a word with a broader meaning than oregano:

oregano: pungent leaves used as seasoning with meats and fowl and in stews and soups and omelets
herb: aromatic potherb used in cookery for its savory qualities

oregano (noun)	herb (noun)
pungent leaves used as seasoning with meats and fowl and in stews and soups and omelets	aromatic potherb used in cookery for its savory qualities
aromatic Eurasian perennial	a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests