oppa (오빠) means an 'older brother'. It is a term used by females. However, it is widely used to address any older male that a girl feels close to, such as a brother, friend, or boyfriend.

just brother

You should call your older brother "oppa". But it's not the only usage. Girls often call older guys they know well by "oppa". You can call you boyfriend like that as well=)))

Oppa means "big brother" It is usually saying by a woman to a man. I hope it could help you in searching for the best answer.