'Oppa' is frequently used by Korean females to address males older than them, but not that much, who they are older brothers, relatively close acquaintances or lover. Mostly, young women use it. Therefore, most Korean men tend to love to be called '오빠' by females younger than them because they're able to feel close to the women and think they're young enough.

On the other hand, adult men are generally called '아저씨'. Not only females but also males can call other men who are not close to them '아저씨'. (http://www.imdb.com/title/tt1527788 , but beware of brutality of this film)

If you're interested in differences between Asian culture and western culture, I would recommend you a book that 'The Geography Of Thought' (Richard E. Nisibett)

'오빠'는 여자가 자신보다 나이가 많은 남자 형제나 남성 지인, 혹은 연인을 부를 때 쓰는 말입니다. 주로 나이가 어리거나 젊은 여자가 나이 차이가 많지 않은 친한 남성을 부를 때 씁니다.(가족간에는 나이 차이가 커도 '오빠'라고 부릅니다.) 그래서, 상대방 남자가 '오빠' 라고 불리면, 그 여자와 친한 느낌이 들고, 또한 자신이 비교적 젊다는 느낌을 받기 때문에 보통 좋아합니다. 이와는 다르게, 일반적으로 '아저씨'는 성인 이상의 별로 친하지 않은 남성을 부를 때 사용되며, 남성이 다른 남성을 부를 때도 사용합니다.

하지만, 이러한 호칭은 주로 사적인 상황에서 쓰기 때문에, 회사와 같은 공적인 상황에서는 자칫 무례하거나 매우 이상하게 느껴질 수 있습니다. 많은 한국사람들이 사람과 사람간에 관계, 사회 구성원으로서의 개인의 위치 및 역할이 중요합니다. 나이를 중요시 하는 것도 같은 비슷한 이유입니다.

For a girl, "Oppa" is not the only way to call her lover. Sometimes, they call "Oppa" with her boy friend's name like "중수오빠" (My wife is two years younger than me and my sister is 6 years younger than me as well. So, I really get used to hearing that in both ways) And a pair of lovers, chiefly for adults, they call "자기" or "(name)씨" each other.

Sometimes, its tone of voice has more meanings than the words literally mean. So to speak, a girl may say "오빠" in a lovely way in contrast with her male sibling.

Thank you for sharing!  I really enjoy hearing about the different nuances that only a native speaker like yourself would know. These days, it seems "oppa" is a word over-run by international fangirls to address their favorite kpop idols on social media.   Also, in kdramas, it seems to mean,  "handsome older-brother figure whom I'm secretly in love with, and plan to marry when I grow up".

That’s interesting to know that “Oppa” is a compliment on age (youthfulness) and degree of closeness. “Ahjussi” is more general and neutral, but does is have a negative connotation of being “too old”?<o:p></o:p>

I think it’s kind of awkward that the same term can be used objectively for an older male sibling, platonically for a familiar male acquaintance, and romantically for a boyfriend/husband.<o:p></o:p>

Since it’s only for younger girls to say “Oppa”, does it imply the older male is protective and doting?  I’m guessing the relationship to younger boys is rather about being wiser and a good example, so they have a different word, “Hyung”,