oh... generally, how many months difference isn't very significant. It sounds a bit funny but I will show example situations. 1. A boy 11 months older than you but he is in the same class with you. Then you don't have to call him 오빠. 2. A boy 2 month older than you but he is in higher class than you, then you should call him 오빠. 3. If you were born in January and the boy was born in December, previous year and one month older than you but you should call him 오빠. 4. If you were born in December and the boy was born in January, the same year with you, 11 months difference, however you don't have to call him 오빠. So what is important is what year you were born in, not how many months older than you. We usually do like this. However, we sometimes don't call 오빠 or 누나 when we agree not to call each other. If you don't want to call a boy 오빠, you don't have to. Just call him his name with 씨 (ssi), eg. Peter 씨, it will work.