Why does this texting quirk bug me so much?

I really hope I don't sound like a B saying this, but I've started getting so annoyed by people who use the word "Oh!" when responding to you in text, as if they're surprised to hear back from you or surprised by your response.

I think it will make better sense in a dramatization... see below.

Friend: "Hey, do you mind if I put your name down for a sub on our intramural softball league? You'll probably never get called on, but just in case!"

Me: "Yeah, of course! Sign me up, coach!"

Friend: "Oh! Awesome! Thanks so much."

Why are you acting surprised? Should I have said no? It's something that irritates me and I feel bad for being irritated by two little letters... Am I completely alone?