“Oh wow, that sounds really interesting.”
You're chatting with the person sitting next to you on an airplane. He tells you that he's a professional writer. You're impressed, so you say this.
Oh wow, that sounds really interesting.
Say this when you hear something surprising, interesting, or upsetting:
A: I met Michael Jackson once.
B: Oh wow. Really? How did that happen?
A: So, it looks like we're moving to San Francisco.
B: Oh wow. You're moving? When?
"Oh wow" is OK in both casual and formal situations.
"That sounds interesting" is a common phrase in English. You can use it in a wide variety of situations:
When someone says something interesting, you can use this phrase with an intensifier like "really" or "so":
That sounds so interesting!
You can also use "That sounds interesting" (by itself, without "so" or "really") if a person says something and you just can't think of a good response:
A: I'm a student at the law school.
B: Oh yeah? That sounds interesting.
You can also use this phrase if you want to hear more about an offer or proposal:
A: I think that you'd be a great actor. Would you consider acting in one of my web videos?
B: That sounds interesting. Where do you film them?