- The word 'oat' in 'My father loves soy, coconut and oat milks'.
I am wondering why the 'oat' is treated as a noun
I am wondering why the 'oat' is treated as a noun
"Oat" is a noun, and "oat milk" could be considered a compound noun. In particular it has the pronunciation of compouds (primary stress on "oat": British Eglish pronunciation) and it is found in several dictionaries, among which the OED, this latter one having recently made the addition.
(CoGEL) [produces/yields] bloodstain, cane sugar, food poisoning, sawdust, hay fever
oat milk: "The oat yields milk"
The mere juxtaposition and the meaning given to the group is what permits to tell it is a compound noun made up of two nouns (compound nouns have to be defined in dictionaries). But, that is not always sufficient: the etymology is sometimes needed; for instace, in "forearm" the part "fore" is not the noun but the prefix "fore-"; in the case of "glow-worm" you have to check the etymology to verify that "glow" is the noun "glow" and not the verb.