Oppa (오빠), a korean word, means
older brother, and can be used for several things;
For a girl to call an older male, to imply she trusts them.
For a girl to call an older male, to show respect.
For a girl to call an older male, in a flirtatious way.
This word's meaning can often be mistaken by daddy instead of
older brother. Although it can be used in a flirtatious way, it isn't a way to call someone "daddy".
Also is what many ARMYs call the members of the k-pop band, BTS. I'm assuming in a flirtatious way.
The member Jungkook (
the youngest) has a great dislike for being called oppa.