No one told me that, I just happen to know it. A guy will let you call him "oppa" if you are close (Best friend or boyfriend). An honorific is something that you can call them if you are close, there are different titles of respect. And some of my friends who have Korean guy friends said that's happen to those guys. They've honestly felt uncomfortable about it. Yes, it can be annoying. It's common sense, how would I have not realized it? Isn't it obvious that they don't like it?