oppa vs adult male

oppa and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
oppa	Yes	No	No	No
adult male	Yes	No	No	No

As nouns, adult male is a hypernym of oppa; that is, adult male is a word with a broader meaning than oppa:

oppa: a male who is older than the speaker, especially used by Korean female
adult male: an adult person who is male (as opposed to a woman)

Other hypernyms of oppa include man.

oppa (noun)	adult male (noun)
a male who is older than the speaker, especially used by Korean female	an adult person who is male (as opposed to a woman)