it explains it in 2nd or 4th chapter idk i dont remember
They said earlier on in the series that in Korea all prostitutes call their clients oppa regardless of whether the prostitute is a boy or a girl so even male prostitutes will call their male clients oppa which is why Je oh calls seong rok oppa
He calls him "oppa" because im previous chapter, the blonde haired rapist dude called himself "oppa" basically referring to make prostitute dude as "like a woman". It seems like he's kind of embracing it in some odd way. (Sorry if you're confused- idk everyone's names)
Oh I understand now. Thank you so much for explaining
Why does he call him Oppa