Actually, when you mean to say oxygen, you mean 1 MOLECULE of oxygen , which happens to be O₂. therefore, we get valency 4 of oxygen MOLECULE, so C ₄O ₄ the ₄ and ₄ cancels and we get CO or carbon monoxide.
In any case, if it is not mentioned as " One atom of Oxygen", we have to take it as O

I hope this clears your doubt.
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