Hello. What is the correct form? - "at one (two, etc) o'clock" - "at 1 (2, 3, etc) oclock" I would be grateful for your help.
The apostrophe must be used: o'clock, not oclock. Whether you use a word 'one' or a figure '1' is your choice. It's not very important, but books would normally use the word.
Yes, the apostrophe there indicates that something has been elided (left out): o' clock is derived from of the clock. You also see this in will o' the wisp, jack o' lantern. You might notice that I put a space after the apostrophe - I am in the minority though.
It is normally considered better English to write small numbers (up to ten) in full so I would always write one o´clock rather than 1 o´clock in any formal writing. When it comes to bigger numbers though e.g.11/12 o´clock, you would usually use the number.
I agree about spelling out the single-digit numbers. In the US, we don't leave a space after the apostrophe.
