> is using “oppa” if ur not korean okay?? like idk american australian etc.

i think this can be a more complex topic
you (and a lot on the internet) are approaching it as a “can a korean person say X word vs can a non-korean say X word” type frame.
which has its merits and probably happens to a certain extent
I’d invite people to consider the following issues as well:

A. to what extent is “oppa” romanticized outside of the korean-speaking-sphere and/or kpop fandom culture? Are people aware of how oppa is used in the language overall, or only narrowly aware of oppa in the domestic fandom culture?

B. do people use some words in isolation? for example “my oppa is so cute” as opposed to “우리 오빠 너무 귀엽다” and how do these interlanguage jargon mixings exacerbate any pre-existing tokenization of the language? Do they further A?

C. Are people aware of the tendency within the korean language to convert/linguistically imagine interpersonal relationships and hierarchies using the language of family and how oppa is part of this larger phenomenom?

etc