used after a number from one to twelve to say the time when it is exactly that hour

It's two o'clock. 两点钟了。

He called me at four o'clock in the morning. 他凌晨4点打电话给我。

used to refer to a time of day when you usually do or have a particular thing

For many mums, the kids' bedtime is chocolate o'clock. 对许多妈妈来说，孩子们的睡觉时间就是巧克力时间。

It's wine o'clock! 葡萄酒时间到了！