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.