Atmospheric oxygen is an important part of the Earth's atmosphere and of the conditions that allow for life on Earth. The atmosphere is composed of approximately 80% nitrogen and 20% oxygen, with trace amounts of other elements. The air we breathe has atmospheric oxygen in it, but the atmosphere also helps to make a protective envelope for the planet. There are two sources of atmospheric oxygen, but the primary way in which the Earth generates oxygen for the atmosphere is through photosynthesis. Photosynthesis accounts for 98% of the world's atmospheric oxygen, while the breakup of water molecules by ultraviolet radiation composes the other 1-2%.

The sources of atmospheric oxygen through photosynthesis are cyanobacteria and plankton in the ocean, and trees on land. The amount of atmospheric oxygen that each source contributes is under debate: some scientists suggest that over half of the world's atmospheric oxygen comes from oceans, for example, while others put the number at closer to one third. What is clear is that the numbers have fluctuated over the centuries, depending on the balance of life on Earth. When the atmosphere was first developing, for example, cyanobacteria contributed the majority of atmospheric oxygen.

Photosynthesis is the process plants and some bacteria use to extract energy from the sun. Water and carbon dioxide are taken up by the organism, and sunlight provides energy for converting these substances into sugars and other energy rich substances that nourish the organism. Oxygen happens to be a highly useful byproduct, and when the Earth is in balance, enough atmospheric oxygen is produced to replace the oxygen that is used.

In the global oceans, a wide range of organisms contribute to the overall amount of atmospheric oxygen. Phytoplankton include diatoms, blue green algae, and other very small but very important ocean organisms. The large numbers of phytoplankton in the ocean are photosynthesizing constantly, creating a steady flow of atmospheric oxygen which can be used by other animals. These organisms are also a food source for marine animals, which in turn provides energy to some land animals and birds which are capable of catching ocean fish.

Forests also contribute to atmospheric oxygen levels, and help to “fix” carbon so that it is not released into the atmosphere to cause an imbalance. Rainforests, subtropical forests, and northern forests all provide habitat, atmospheric oxygen, and beauty. Global deforestation has raised concerns about atmospheric oxygen because it may lead to a decline in oxygen levels and it certainly leads to an increase in atmospheric carbon dioxide, which scientists agree is unhealthy.