哥哥

English says 'big brother', so learners bolt 大 onto 哥哥. But 哥哥 already includes the idea of older. In Chinese, 大哥哥 is not how you refer to your own older brother. It sounds like a child calling an older boy. For your own brother, just say 哥哥. If you have multiple older brothers, use 大哥, 二哥, etc.

English just says 'brother', so you must retrain your brain. Chinese kinship terms always show age order. 哥哥 is an older brother. 弟弟 is a younger brother. You cannot use them interchangeably. If you have one of each, you say 哥哥和弟弟. If you only have a younger brother, you say 弟弟, never 哥哥. “哥哥和弟弟，姐姐和妹妹 也是我家人，也是好朋友。” (My older and younger brothers, my older and younger sisters — they're my family and also my good friends.) ── [《我的家人（2）》](/read/my-family-members#s9)

哥哥 is for your actual older brother or very intimate talk. It is not a generic 'bro' for coworkers. If you want to be friendly with an older male colleague, use 哥 after his surname, like 王哥. That sounds casual but not romantic. For strangers, use 先生 or just avoid the title.