When you talk about the time, you don't always need to add "o'clock". You can also just say the number:

A: When's the meeting?

B: It's at three.

Adding "o'clock" when it's not necessary makes the sentence sound more formal:

The meeting is at four o'clock.

Adding "am" or "pm" to the time when they're not necessary makes it even more formal:

The meeting will begin at nine o'clock a.m. Please arrive ten to fifteen minutes early.

This phrase appears in these lessons: