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
" sang chan please help me to put on this band aid"
"Ye Jin ah! How dare you speak my name"