(1) Brother. This is the simplest use of oppa: the blood relative. This can be a brother or cousin a girl, as long as it is male and over. Oddly enough, oppa can even mean a twin, if born before the sister.
The older guy. The term oppa can also extend to the guys who are not blood related, but the girl feels a relation some closeness. When a girl is small, this is pretty easy to tell all the older children.
When the girl begins to grow however, has more options for calling things older kids and therefore the use of "oppa" is used with discretion.
A guy who called oppa being with a girl who is not related, also acquires an implied responsibility as generally watch over her and making sure they do not get in trouble. He is the one who could teach social drinking - but also the type you'd better withdraw it when she has drunk too.
The romantic oppa: A girl often called her boyfriend "oppa."
So you can imagine that the word now takes all these implications. You're not only the protective big brother. Now oppa is the object of romantic affection - the manly man.
