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.