red osier vs osier

red osier and osier both are nouns.

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

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

red osier: 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 osier (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
common North American shrub with reddish purple twigs and white flowers	flexible twig of a willow tree