willow oak vs oak

willow oak and oak both are nouns.

Word	Noun	Adjective	Verb	Adverb
willow oak	Yes	No	No	No
oak	Yes	No	No	No

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

willow oak: medium to large deciduous oak of the eastern United States having long lanceolate leaves and soft strong wood
oak: a deciduous tree of the genus Quercus; has acorns and lobed leaves

Other hypernyms of willow oak include oak tree.

willow oak (noun)	oak (noun)
medium to large deciduous oak of the eastern United States having long lanceolate leaves and soft strong wood	a deciduous tree of the genus Quercus; has acorns and lobed leaves
	the hard durable wood of any oak; used especially for furniture and flooring