Oppa

Oppa is used in Korean culture by a female to a male, who is older than her by the boundary of 10 years. Female's use this title for their older brother, older male friend, or even in a flirtatious way.

something you do not say to jeon jungkook.

Oppa is a Korean term usually meaning an older male such as an older brother, male friend, and can also be used while Flirting
Ways people say Oppa

Oppa is a term that is used in Korean by a girl to call a man who is older than her, but by less then about 10 years. It is a term used traditionally in respect to ones elders
An oppa can be an:
Real biological older brother
A male friend who is older than the woman
An older male a girl is trying to flirt with

A respectful Korean term used by younger females toward older males; such as an older male friend. Also, is sometimes used in a slightly flirtatious way.

Something you should NEVER call Jungkook

A respectful Korean term used by females to call older males such as older male friends or older brothers, but now with the Hallyu kickin' in, people are using it being as annoying as the Japanese, "Kawaii" wave.

Usually used in the annoying way by crazy kpop or jpop fangirls.