What is O’clock ?
O’clock is (adv) used with numbers to show the time Get up it’s 7 o’clock. We never open the shop before 10 o’clock. By 2 o’clock in the morning everyone was asleep. (NOTE: O’clock is only used for the exact hour, not for times which include minutes. It can also be omitted: We got home before eight. or We got home before eight o’clock.)
source: Easier English, Student Dictionary Upper Intermediate Level