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O’clock is used with times on the hour, whether those times are written as words or in numbers (e.g. three o’clock or 3 o’clock, but not 4:30 o’clock).

  • Mischa said he would arrive about four o’clock in the morning.
  • When the Peace Tower bells rang at 10 o’clock, the veterans assembled for the parade.

Never use a.m. or p.m. with the expression o’clock.

  • Sarah called at 5 o’clock (not at 5 o’clock p.m.).