Oppa, technically, means big brother. It is used by a female to address a man she is close to that is older than her. It is also pretty much the first Korean word that any Kpop or Kdrama fan learns. Why? So if we meet our bias we know what to call them! Super Junior members Donghae and Eunhyuk even made a song about it.
Here in the US we don’t really have an equivalent word. When I have seen subtitles try to translate it they usually either say brother or honey, depending on who is using it. That is because it can really be used either way. It is what a little girl would call her older brother and it wouldn’t be flirtatious in any way. But if you hear a 20 something girl calling a guy she is interested in or her boyfriend Oppa that is most likely flirtatious. There is also the male equivalent, Noona. This is used much less often in a flirtatious way, though “Noona romances” are becoming much more popular in dramas. My personal favorite is Flower Boy Ramyun Shop.
This sort of word is actually really common in all of Asia. Pretty much every Asian language I have looked up has words that designate older brother and older sister, and normally a different set for each gender. In Korea if you are a man you would call an older woman Noona or an older man Hyung. If you are a woman an older man is Oppa and an older woman is Unnie. Some languages have it separated out the same way for younger people but in Korea you call all everyone younger Dongsaeng. Now, if you go to Korea don’t just walk around calling everyone this. They are more like endearments; you do not call strangers these things!
Unless they are celebrities, then it is completely okay. Love you T.O.P. Oppa!