Are you trying to build a underwater habitat, or maybe a space station? Maybe you just want a cleaner indoor environment. You can build a CO2 scrubber with algae. This will remove CO2, produce Oxygen, and some types of algae (spirulina) are a high nutrient food source (as well as a fuel source).
Another way to produce oxygen (and hydrogen) is electrolyzing water, but it’s a very inefficient process.