What part of speech is the word oak?

The word "oak" can be several parts of speech, depending on how it's used in a sentence:

* Noun: When referring to the tree itself. Example: "The oak stood tall and proud."

* Adjective: When describing something made of oak wood. Example: "The oak table was a family heirloom."

Let me know if you have a sentence in mind and I can tell you which part of speech it is acting as!