In English, many words can fill more than one function of speech (but of course, many are limited to only one function of speech).
Oak can be a noun or an adjective.
An adjective generally does the job of answering the question "What kind of X?".
While English adjectives can sometimes come after the noun, usually they come before, and adjectives answering "What kind of X" almost always come before the noun.
@NES provided an answer that talked about compound words. Compound words in English can be separated by nothing (e.g. flowerpot), a hyphen (e.g. full-fledged), or a space (e.g. ice cream).
In the case where they are separated by a space, one word is obviously technically a modifier, but combined they mean something different than the original two words. Continuing with the example of ice cream - while ice and cream are used to make ice cream, the end result is not really a type of ice or a type of cream. It's a new thing. You'll probably might occasionally see people spell it incorrectly as one word for this reason, icecream.
Whereas with oak tree - it's still a tree - oak is qualifying the type of tree, thus this isn't really a compound word.
2This question is not a duplicate, since it asks about a compound word. oak tree is a compound word and has its own entry in the OED. So do cough drop and cough medicine, but cough trouble is not recognized as a compound word. – None – 2015-12-15T14:59:30.003
1Though "oak" in "oak tree" is very often said to be an adjective this is highly imprecise grammar terminology. "Oak tree"is a compond noun of the type noun + noun with "tree" being the main element and "oak" the subelement. English grammar should have a special term for "oak". I never use "adjective" for nouns that are subelement of compound nouns as it is confusion of grammar terms. Adjective is a word class, but oak is an element of word formation and as such quite different from normal adjectives. – rogermue – 2015-12-15T18:50:25.120
1There is "a black bird" and "a blackbird". It is nonsense to say "black" is an adjective in both nouns. That is very imprecise grammar terminology and I would not recommend it. "Black" in blackbird has quite a different function from a normal adjective and a special term would be necessary. – rogermue – 2015-12-16T06:56:29.447