Questions regarding consumption and production of oxygen
A breathing mix containing oxygen is necessary for humans, and all manned space-travel must therefore deal with that. On a space-flight, oxygen is a consumable, unless you find a way of recycling it or produce more of it. A common method is to electrolyse water to liberate the bounded oxygen. Producing oxygen by the means of photosynthesis has also been considered, but have not yet been tested further than closed ecosystems experiments on Earth (Bios 1,2 and 3, Biosphere 2).