Always use the shortened form, "o'clock."

Is it nine o'clock or 9 o clock?

You use o'clock after numbers from one to twelve to say what time it is. For example, if you say that it is 9 o'clock, you mean that it is nine hours after midnight or nine hours after midday. I got up at seven o'clock this morning.

How do you write o clock?

At the beginning of each hour, we use the term “o'clock”. For example, “It's one o'clock” (1:00). Or “It's four o'clock” (4:00).