red willow vs osier

red willow and osier both are nouns.

Word	Noun	Adjective	Verb	Adverb
red willow	Yes	No	No	No
osier	Yes	No	No	No

As nouns, osier is a hypernym of red willow; that is, osier is a word with a broader meaning than red willow:

red willow: Eurasian osier having reddish or purple twigs and bark rich in tannin
osier: any of various willows having pliable twigs used in basketry and furniture

red willow (noun)	osier (noun)
Eurasian osier having reddish or purple twigs and bark rich in tannin	any of various willows having pliable twigs used in basketry and furniture
	flexible twig of a willow tree