brash oak vs oak tree

brash oak and oak tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
brash oak	Yes	No	No	No
oak tree	Yes	No	No	No

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

brash oak: small deciduous tree of eastern and central United States having dark green lyrate pinnatifid leaves and tough moisture-resistant wood used especially for fence posts
oak tree: a deciduous tree of the genus Quercus; has acorns and lobed leaves

Other hypernyms of brash oak include oak.

brash oak (noun)	oak tree (noun)
small deciduous tree of eastern and central United States having dark green lyrate pinnatifid leaves and tough moisture-resistant wood used especially for fence posts	a deciduous tree of the genus Quercus; has acorns and lobed leaves