> isn't "diatomic oxygen" the stuff we breathe every single minute and need to survive?

I think they're referring to oxidative stress [1] caused by cellular respiration.

[1] https://en.wikipedia.org/wiki/Oxidative_stress

Yes and it wasn't sarcastic, both things (what you said and what they said) are simply true. I think their point was not to be alarmist like you should stop breathing, but simply that everyone knows the one thing and most people don't know the other thing, and it gives scale or context to the "you don't want extra ozone".

Part of aging is the result of oxidation of DNA over time and as cells reproduce.

The other posters here are right. I just wanted to point out the odd beauty in the fact that the fuel that makes our biology possible is also one of the things that is killing us. If you get a chance, read up a little bit on redox reactions and oxidative stress and take a moment to appreciate that.

We harness the energy of oxygen. It's the fuel that powers us. But it's also so reactive that it's constantly damaging our DNA and intracellular components. Over time it ages us, causes cancer (daily - our biology fights back!), and will ultimately be part of what wears us down and kills us.

Look at oxygen as a very reactive and energy abundant fuel. And then consider its abundance in our atmosphere. And then how it powers human locomotion, human biochemistry. And how it creates free radicals and chain reactions that strip our DNA and mutate base pairs. And all the tens of thousands - no, countless more - interactions and reactions it's having all throughout our bodies at all times. And how our biology evolved compensation measures to keep those deleterious effects at bay for as long as possible - as long as necessary - to enable reproduction.

Our biology is utterly awe-inspiring when you think about it. An incredible machine molded by our gravity well and abundantly available energy. Not just fighting against entropy, but actively sailing its turbulent energy gradients.