The oxygen we breathe comes from a lot of different places. The largest source of oxygen comes from organisms (living things) that can perform photosynthesis, which is a word that describes the process in which a living thing uses sunlight, carbon dioxide and water to make sugars and oxygen. After sugars and oxygen are produced, the organism uses the sugars as energy, and sends the oxygen out as a "waste" product. There are many things that perform photosynthesis, especially plants and organisms in the ocean. As to the source of the carbon dioxide, it can come from humans when we burn fossil fuels (gasoline and so on); carbon dioxide can also be released into the atmosphere by volcanoes and from rocks that have carbon. Together, the amount of carbon dioxide in the atmosphere and the number of organisms that can perform photosynthesis play very important roles in the amount of oxygen available in our atmosphere for breathing and other processes (such as lighting a fire).

The oxygen (O₂) of Earth is primarily a byproduct of photosynthesis. Photosynthesis is the process performed by plants and plant-like organisms like phytoplankton which converts sunlight and CO₂ into food. To these organisms, the O₂ is a waste product that is released. Although land-based plants, such as rain forests, are the focus of highly-publicized conservation efforts, the photosynthesizing organisms in the ocean contribute far more O₂; up to 85% of the oxygen in our atmosphere comes from these ocean-dwellers. (Delving a bit deeper, the photosynthesizing part of these organisms is the chloroplast. Chloroplasts are thought to be the descendants of an ancient cyanobacterium.

It was cyanobacteria which first began producing oxygen, billions of years ago. The chloroplasts in plants developed after cyanobacteria formed symbiotic (mutually beneficial) relationships with early eukaryotic cell which engulfed ("ate") them.)

Plants, algae, and some bacteria make sugars out of carbon dioxide, water, and light. In the process of making these sugars, they produce oxygen as a byproduct. This is where all of our oxygen comes from.