Here's why:
* Interjections are words or phrases that express strong emotion, and they are often used independently of the rest of the sentence.
* "Oh" is often used to express surprise, realization, or a range of other emotions like joy, pain, or disappointment.
Here are some examples:
* Oh! I forgot my keys.
* Oh no! I spilled coffee on my shirt.
* Oh, that's interesting.
While "oh" can sometimes be used as an adjective (like in "Oh, what a beautiful day!"), its primary function is as an interjection.