I was reading a manga and a high school student goes to a university, then talks to a person and says:
After the small chat the other person that seems to be a student from that university says in her thoughts:
So I have some questions:
Why with the おい she realizes it was a high schooler? Or it was simply weird?
What's the difference among おいっ and おい?
When, how, to whom, when not should I use this interjection?