It can be said that oxygen is a poison which humans
and other organisms just have adapted to tolerate to some extent by developing antioxidants, which work to prevent the damage from free radicals.
In fact, a long long ago there was a time when Earth's atmosphere didn't contain oxygen at all and everyone was happy. Then appeared organisms with the capability to produce oxygen and the oxygen content of the atmosphere started to rise. This is called the oxygen holocaust. Many, many species died and became extinct as the oxygen level rose before they could adapt.
When the oxygen level had finally stabilized, the remaining organisms had developed protection against oxygen and it later became a vital gas for a large part of all life on Earth.
Meanwhile, humans have inherited the resistance to oxygen poisoning, but in larger partial pressures that do not occur naturally the oxygen still becomes toxic. The limits used to be higher, but nowadays most organizations place the upper limit to 1.4 bar of ppO2 when working and to 1.6 bar in rest.
These limits are very easy to violate by breathing the wrong mix underwater. Technical divers usually carry a decompression gas with high oxygen content in addition to their bottom gas. Accidentally breathing the decompression gas (which typically has a 50 to 100 percent oxygen content) when deep underwater will put the diver into convulsions after a few breaths. This (breathing the wrong gas) is in fact the single most common cause of death in technical diving.