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.