be on somebody （口语中使用）某人来买单
It's on me.
我买单。
The drinks are on me！
饮料我来买单！

What's somebody on? (口语中使用）指某人行为异常，像磕了药，吃错了药一样
What are you on?
你吃错什么药了？你嗑药了？

having something on (informal) （预先安排的事）将发生
I haven't got anything on tomorrow.
我明天还没有安排。

be on for something(informal) 想要做某事
How many of you are on for a drink after work today?
有几个人下班要去喝一杯呀？
Is anyone on for a drink after work?
有人想下班后去喝一杯吗？

I'm on it. 我马上做。

be/go/keep on somebody(informal) 抱怨，唠叨，发牢骚
Stop keeping on about it!
别再唠叨那件事了。
I wish you wouldn't go on at me the whole time.
我希望你不要一直跟我抱怨。

you're on (口语中使用）赌就赌吧。
I bet you're wrong.
我打赌是你错了。
You're on.
赌就赌吧。