The common convention is like this. one brother - 오빠 two - 큰오빠, 작은오빠 hree - 큰오빠, 작은오빠, 막내오빠 four - 큰오빠, 작은오빠, 세째오빠, 막내오빠 five - 큰오빠, 작은오빠, 세째오빠, 네째오빠, 막내오빠 You see that 막내 means the youngest. One's youngest son is 막내아들, youngest daughter 막내딸. Similarly, 막내고모 is youngest aunt on father's side, 막내이모 youngest aunt on mother's side. But all this is mostly a thing of the past now. Nowadays Korean families have shrunk to one or two kids on the average. So people won't have much need for these words, which is kind of unfortunate :-)

If you have many older brothers, you can call 큰오빠 or 첫째 오빠 for oldest brother,둘째 오빠, 세째 오빠 etc according the rank for middle older brother and 막내 오빠 for last older brother.

I'm not a native! but I guess all of them are 오빠 regardless your age gap =D