There's some misconception. Korean girls call their boyfriends only if they are older. and calling "오빠" brings more intimacy, just like calling a friend "bro".

In early 80's, some girls started to call their older boyfriend 오빠 while other girls were still calling their b/f "(name)씨". We can take this to mean calling 오빠 breaks any formalities between a couple. Also, one's 오빠 will try to protect his younger sister as much as he can and help her in some way when she's in trouble. Calling one's older boyfriend 오빠 shows those girls expect their older boyfriend to act that way.

oh really? i've never thought the reason for it. by the way how do u know that?

I love to be called "오빠"(it sounds cute and sweet), and I agree with Jesse Choi.:) In a family, I think there is no reason because they are already close enough. If you really want to be close or show intimacy to a guy, try to call him "오빠". (It could be "a magic word" like "please~~".)

Its strange, isnt it? Korean girls call their real brother '오빠' ; on the other side, they also call their boy-friend the same '오빠'/brother and they have physical relationship with one whome they call brother... What is that culture ? Can Americans or any country's girls do the same ? I mean call somebody brother and have physical relationship with him ? I think its disgusting ?