What is the major source of free oxygen in the atmosphere?

Would it be trees?

No, trees are not the major source of free oxygen in the atmosphere. The major source of free oxygen in the atmosphere comes from a process called photosynthesis, which is performed by plants, algae, and some types of bacteria. During photosynthesis, these organisms use sunlight, carbon dioxide, and water to produce oxygen and glucose (a type of sugar). This process predominantly occurs in the Earth's oceans, where marine plants and microscopic phytoplankton are responsible for approximately 70% of the oxygen production. The remaining 30% is provided by land plants and trees through photosynthesis. While trees contribute to the overall oxygen production, they are not the primary source.