Oxygen is the gas that acts as a waste product of photosynthesis. During this process, plants convert carbon dioxide and water into glucose and oxygen using sunlight as energy. Photosynthesis takes place in the chloroplasts of plant cells, where light energy drives the chemical reactions. The oxygen produced is released into the atmosphere, which is essential for the respiration of most living organisms. This process not only sustains the plant itself but also contributes significantly to the oxygen supply on Earth, thus supporting life.