The answer given is close but not completely correct.
오빠 oppa, means older brother for girls. This can be in the literal sense that someone is their brother. However, it is also used by girls to older men (even if they are not their boyfriend) It's a respectful way to name some body
형 hyeong (hyung) Is what men call their older brothers (and older males)
언니 eoni is older sister for females. Again this can mean actual sister, or to an older girl.
누나 nuna is what guys call their older sisters and again girls who are older than them
so to sum up
오빠 what girls call older guys
형 what guys call older guys
언니 what girls call older girls
누나 what guys call older girls