It is objectively wrong to say 14 o’clock, because “o’clock” refers to the orientation of an analogue clock.

Saying “it’s nine in the morning” is redundant in a 24 hour system, because nine would never be anything other than that.

To say 'it’s nine hundred" reduces the ambiguity slightly (because you can’t really say o’clock).

If you simply say “it’s nine” then other people might ask “what’s nine?”

Is it “nine past nine”? Or are you telling me “no” in German?

Nine hundred is pretty clear, but not to our primitive ears