I would say the address words for people (up to several years) older are, from distant to close: (using 이정훈 as an example) XX 씨 -> 선배님 / 정훈 선배님 / 이 선배님 -> 선배 / 정훈 선배 / 이 선배 -> 정훈 오빠 (or 언니 if female) -> 오빠 (or 언니).

As a foreigner, I think always asking the person if you should call them 오빠/언니 is a very polite and safe way to approach the situation. Usually, Koreans are forgiving to foreigners in this regard. If you're uncomfortable doing that, the next best thing to do is to soften the use of a straight-up 오빠/언니 by saying his/her name in front of it (i.e. "성용 오빠"), and analyzing their response. Of course, I'm assuming at this point you have somehow broken the ice by addressing the person with name-씨 or some other equivalent, so don't do that if you're still at a stranger acquaintance relationship. If that is still too scary, then you can just keep using name-씨, and he/she will eventually correct you as you guys get closer and he/she gets uncomfortable with you addressing him/her as such. But, you should know that I am American-born with immigrant parents, so you may want to put more weight to an actual Korean answer for this question.