Did you record that? In that example, by doing a rapid version of “6:09”, you’ve pronounced both of the phrases with a schwa, and therefore neither of them with a full “oh” sound. So, exactly the opposite of what you claimed.

In any case, with “6:09” you could (and would certainly commonly hear others in your country) very naturally fully pronounce the “oh”, whereas with “o’clock” it would sound unnatural, like you are over-pronouncing it.

Look in any dictionary and see that the first syllable of “o’clock” is just /ə/, whereas the word “oh” is /oʊ/ or /əʊ/, or for an Australian dictionary maybe even /əʉ/ or /æʉ/. Whatever it is, it will not be the same as “o’clock”.

I feel like I’ve really patiently tried to explain this to you, even as you have been rude and insulting at every turn. At this point, if you actually care to understand, I suggest you google “vowel reduction” and sort it out for yourself.