Most of the planet's oxygen is produced through photosynthesis in plants, particularly by phytoplankton in the oceans. These microorganisms are responsible for generating a significant portion of the Earth's oxygen supply through the photosynthetic process, where they convert carbon dioxide and sunlight into oxygen and glucose.
While plants, including trees and terrestrial vegetation, also contribute to oxygen production, phytoplankton is often credited with providing around 50% to 80% of the world's oxygen supply. In contrast, animals and humans breathe in oxygen and do not produce it; instead, they utilize it for metabolic processes.