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Onni (언니) - A girl's older sister. Can also be used by girls to refer to random girls older than them.

Oppa (오빠) - Korean for a girl's older brother (a boy's older brother uses a different word). This word is also used by girls to refer to other boys slightly older than them. You will hear this term a lot on the subway as college aged girls phone or speak to their boyfriends. I am never called oppa since my girlfriend is older than I am.