Um...I'm sorry, but I have to say this and please don't be offended. You just aren't suppose to call a guy "oppa" if you don't know him. You can do it if you are close, best friends forever with him, I don't care. But just calling a random guy that, it's weird. Even Korean guys think it's weird. They won't tell you, but they get uncomfortable. You just don't do that. It's an honorific for a reason, you earn the privilege to call them that by becoming friends with them. I'm sorry, but I just thought I'd let you know. :/