The primary source of free Oxygen in nature is because of the photosynthetic action of green algae which is found in abundance on the planet. An estimated 95% of atmospheric oxygen is produced by the green vegetation on the planet, most of which is contributed by the green algae. Plants consume carbon di oxide to produce energy for sustaining themselves which is used to produced carbohydrates and oxygen is a by product of those energy reactions. They use solar radiation for that process.
