Why is there oxygen on Earth and not in space?

Answer

Oxygen (O₂) as a gas indeed does not occur in space, but does occur in the atmosphere of the Earth. The Earth’s atmosphere is therefore something very special, definitely not a “normal” atmosphere. A “normal” planetary atmosphere, such as on Venus or Mars, is a carbon dioxide (CO₂).
Oxygen is a very reactive gas. If it were not constantly produced, the oxygen would gradually disappear from our atmosphere. But how do we get the oxygen in our atmosphere?
By photosynthesis. Plants use carbon dioxide from the atmosphere as an energy source. But in doing so, the plants produce a waste product: oxygen. The plants therefore continuously pump that gas into the atmosphere.
More than 2 billion years ago, blue algae started photosynthesis for the first time in Earth’s history. This has led to a massive “pollution” of the atmosphere at that time by oxygen. Oxygen is therefore the first large-scale atmospheric pollution caused by living things. Today it is humans who are pumping carbon dioxide back into the atmosphere en masse.
Oxygen in the atmosphere is not only important to us because we breathe it in, but high in the atmosphere oxygen also reacts with the harmful UV radiation from the Sun. This is how ozone (O₃). This ozone layer protects us against harmful solar radiation. Otherwise life on land would be virtually impossible.
Oxygen in the atmosphere is thus a typical feature of a planet where living organisms perform photosynthesis. Suppose we now discover a planet somewhere in the universe with oxygen (or ozone) in the atmosphere, this would be a strong indication that there is life on that planet …. but until today we have no such planet yet found it.