I eat lunch at one o'clock every day.
Put your toys away by one o'clock.
If you worked for me, your shift would start at one o’clock in the afternoon and end at three o’clock at latest. Sound good?
When it's one o'clock it's one o'clock. It's time to eat lunch, or take a nap, or something.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.