From my understanding it's a Korean slang word used by girls towards older men with whom they are friendly. I'm not sure if it's sexual or not. It literally means older brother.
I think if a girl has a platonic friend who is older, she may refer to him as Oppa. It's a term of endearment.
At least that's my understanding of it, limited as it may be. I'm not Korean, and I'm not sure if I'm getting it. That's how it was explained to me.