(Familiar) one's older brother; a casual/affectionate way to refer to or address an older brother.
お兄ちゃん、宿題手伝ってくれる？
Big brother, can you help me with my homework?
子どもが遊びに来ると、みんなお兄ちゃんに甘える。
When kids come to play, they all cling to (act spoiled with) the older brother.
彼女は彼のことを『お兄ちゃん』と呼んでいるが、年の差はあまりない。
She calls him 'onii-chan,' although there isn't much age difference between them.
A familiar vocative used to address a young man (e.g., children calling a man 'sonny' or 'mister'); often used as a casual vocative.
遊園地で迷子の子が『お兄ちゃん！』と大声で呼んでいた。
A lost child at the amusement park was shouting 'onii-chan!' loudly.
商店街で若い男性に『お兄ちゃん、このクーポン使えますか？』と聞く人がいる。
On the shopping street some people ask young men, 'Hey, can I use this coupon?' (using 'onii-chan').