Which word in the following sentence is an Interjection?

Mark said, “Oh I didn’t realize you were coming over today”

Said
Oh***
You
Over

Right.

The word "Oh" is an interjection in the sentence.

To determine this, you can recall that interjections are words or phrases that express strong emotions or sudden exclamations. They are often set apart from the rest of the sentence by an exclamation point or a comma. In this sentence, "Oh" is a sudden exclamation expressing surprise or realization.