No, it's not idiomatic to say or write six-thirty o'clock. Six o'clock, six-thirty, seven o'clock, seven-thirty, etc. I'm not sure why this is, but that's how it's said.
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GuestIs is proper to say: dinner is at Six Thirty O'clock?
Doesn't o'clock mean "on the clock" or "on the hour"?
I think it sounds awkard, but I'm in the process of printing a formal invite, and a friend keeps saying it's at six thirty o'clock!
Help!
pedant"O'clock" = "on the clock" = "on the hour."O'clock actually comes from of the clock.