"9:00" can be read as either "nine o'clock" or "nine". So, adding "o'clock" is redundant.

In writing you could clarify, if necessary, by stating ‘am’ [morning] or ‘pm’ [afternoon or evening]. In speaking, we often do say ‘ . . nine o’clock’, although you can also say ‘It starts at nine’.

You could also add AM or PM instead of o'clock. For example 9 am or 9pm.

Hi, yes, you are correct. Sometimes we add in the morning or in the evening if not clear.