One thing to note is that humans(and animals) will die from CO2 poisoning long before they run out of oxygen in an enclosed space. So yeah, CO2 scrubbers/absorbers are necessary.
Options vary - as Roadking mentioned, there's several varieties. The differences involve cost, efficiency, and how easy they are to regenerate. Kinda like the moisture removal systems many of us use in our gun safes - Many you can simply bake off, some would require more complex chemical regeneration. Some absorb more moisture than others.
Of course, another problem we have is that we don't actually have a biological low O2 alarm. What we have is a CO2 buildup alarm - we can asphyxiate in a low oxygen without noticing if CO2 is also low.
It's the core for my idea about implimenting the death penalty - seal convict into relatively air tight room, then simply fill with nitrogen with a little positive pressure. The other option would be to use a more air tight chamber and simply recycle the air, passing it through a scrubber.
