Why "Oppa" ??

한국 친국가 있어요? (Do you have a Korean friend?) How do you call your friend?

In Korean, family titles are used to refer to non-family members outside your family.

친구 only refers to someone your same age. Older friends are not 친구 in Korean.

If you are a woman, older females should be called 언니 and older males should be called 오빠, just like the way you call your own family members.

[KBS World] Pit-A-Pat Korean (두근두근 한국어) - Episode 3