Oppa (오빠) is a Korean honorific. As defined on the Busy Atom website, it is “a gender specific term and used by a girl (or female) to call her elder brother (or brother like figures). The following 3 major definition summarize who oppa is as of today in Korea:

a girl's big brother (including male cousin who is older than the girl)

a girl's male friend who is older than the girl and in very close relationship including romantic relationship

a fangirl's male celebrity who is older that the fangirl.”

Since this term is part of a really intricate system within the Korean culture, it is frequently seen in song lyrics and variety shows. Its use is also widespread among the K-pop fandom, including the Western fans.