Plants release oxygen during photosynthesis by splitting water molecules (H2O) into hydrogen ions (H+) and oxygen (O2) in the presence of light. This process occurs in the light-dependent reactions of photosynthesis, where the energy from sunlight excites the chlorophyll in the plant, leading to the splitting of water molecules. The oxygen released is a by-product of this process and is released into the atmosphere, while the hydrogen ions and electrons are used to produce ATP and NADPH.