How do you feel like when you hear that oxygen is actually poisonus?

Apparently, there's something called the "Free-radical theory of aging" which says that organisms age because cells accumulate free radical damage over time. A free radical is any atom or molecule that has a single unpaired electron in an outer shell.

Think about some kind of oxidative damage.

You may recognize it as Rust or Corrosion wich happens to some metals when left outdoor.

And this:

When you lave a cut apple exposed to oxygen. Another example of oxidation.

There are different conditions for every element to get oxidized. Exactly the same happens inside your mitochondria when it turns glucose into ATP, and when this occurs the reaction results in a left over electron, or many left over electrons, which are referred to as Free Radicals.

Your body naturally produces antioxidants, to prevent the damage caused by these electrons hungry cells, but at the end the damage aggregates into a final transition… death by old age.
Antioxidants are like “Bro..stop bothering him. Here, I got an electron for you.” (represented in the picture below)

But please don't stop breathing, that will kill you a lot faster.