'오빠' actually , this has more meanings, check here: https://youtu.be/tB-2Hv5eyJM

Yes, they are both correct. 오빠 means "older brother" when a girl says it. 형 means "older brother" when a guy says it. As Emily said, you don't have to be related to use it. In fact, it is a sign of closeness between friends. We use 동생 to mean "younger sibling" (male or female - there is no distinction unless you add 남+동생 for a guy, or 여+동생 for a girl).

oppa means 'big brother' but you dont have to be related in order to say it hyung 형, and oppa 오빠, both mean "older brother." but only boys can use hyung and only girls can use oppa. its out of respect, i think. if a woman knows a man somewhat older than her, she can call him oppa. hope it helps :)

When women call the older brother , ' oppa ' in Korean 오빠 If man's age is too old, women call ' a ju see ' in Korean 아저씨 ※ When menl call the older brother , ' hyung ' in Korean 형