Sorry for the long response time, I was traveling all day yesterday.
All possible because we breathe and stay alive.
And this is where your defnition is arbitrary. We are alive because countless things are just the way they are. If plants didn't use photosynthesis, there would be no O2, so we really owe our existence to plants. But, then, if it were for hydrogen and gravity, there wouldn't be light to power the plants. If it were for the strong nuclear force and neutrons, neither O2 or hydrogen would exist.
It appears you are justck picking one thing out of the list of things necessary for humans (and all life) to exist, breathing. It is most certainly NOT only because oxygen exists that we do. It's one of many things.
I'm not getting why that one thing of many is the choice. It's arbitrary.