oxygen (o2) is formed in photosynthesis as a direct result of
Light-driven splitting of water during oxygenic photosynthesis
Photosynthesis and respiration
n nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesi
UNANSWERABLE Using the energy of sunlight to produce oxygen from water and carbon dioxide. Photosynthesis uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen Oxygen
Introduction Introduction
 Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. 
wiki:25633213
Oxygen [SEP] Photosynthesis and respiration
In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth, and the rest is produced by terrestrial plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year. A simplified overall formula for photosynthesis is: or simply