The term (오빠) or oppa is a phrase commonly used when a girl is referring to an older guy. It is not strange or impolite. You can call any guy older than you oppa. Although if there is a dramatically large age difference I recommend using the suffix sshi (씨), which is more respectful. You can also use the suffix sshi for anyone regardless of gender.

Calling someone older than you by their given names is inappropriate and impolite in the Korean language, so 'oppa' is used when you call a guy who is older than you. But you can only call a guy 'oppa' when he is around your age. You can't call a man 'oppa', for example, who's the same age as your father. It is also considered inappropriate in a formal situation. Plus, 'oppa' can be a common noun that refers to a woman's older brother. (I have an older brother = I have an oppa)

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Please see the discussion I posted: Why do many guys like to be called "Oppa" https://www.italki.com/discussion/140195

Hello there. I've never heard a girl call a guy oppa. That name, oppa, sounds like an affectionate name someone might call their grandmother.