Oxygen is oxidizing, that why we breathe it. It gives us energy by reacting with food.
Now to the right part: oxygen once in history WAS poisonous. It happened when photosynthetic bacteria began doing it, and other cells died. Survived only that who had mitochondria. And so it continues, to now.
If anyone wants to, I have new roleplay server on Discord. It is multiple mythologies toss together, which accept each other. It is in alternate universe, similiar to our own, but in some points different. And the gods are way nicer than.in real life myths. NSFW content is strictly prohibited...