The primary source of oxygen on Earth is photosynthetic organisms, specifically plants, algae, and cyanobacteria.

Here's why:

* Photosynthesis: These organisms use sunlight, water, and carbon dioxide to produce their own food (glucose) and release oxygen as a byproduct. This process is called photosynthesis.

* Oxygen Production: The oxygen we breathe is a direct result of this photosynthetic process.

While some other organisms produce small amounts of oxygen, the vast majority of the oxygen in our atmosphere comes from photosynthetic organisms.