The machine's #1 goal is survival.
To survive the machines use a form of fusion, combined with the energy from the pods.
Fusion reactions turn lighter atoms (like hydrogen and helium) in to heavier ones (like oxygen, carbon and metals)
There is already an abundance of oxygen on the planet that was not used due to plant and human life being mostly killed off.
The ocean produces oxygen due to various organisms that get their energy from chemicals in ocean floor volcanic vents and other nutrients... as well as many bacterias.
These alone would probably not produce enough oxygen to cover usage requirements for the bluepills and foetus fields.
So you think perhaps the machines produce oxygen and pipe it to the humans in their control. However oxygen would need to be piped to each foetus pod. Pipes get blocked, rust, corrode or otherwise deteriorate. The best use of the machine's time would be to produce oxygen, cool it (so it is heavy and stays near the ground longer before disipating) and blow it out over the fields in low concentrations (high concentrations kill bio-organisms and corrode metals and plastics). Zion has it's own water and air processing machinery (most likely powered by geo-thermal energy from the earth's core).
Oh, by the way... the sky is blackened right around the globe. "The second renaisance" from the animatrix tells us this. There are low levels of light, enough to support the organic life I mentioned earlier, but not enough for the machines' solar energy requirements. Also there are many sources of artificial light in and around the machines' and their constructions which could support organic life (like in "the bread feast" story mentioned earlier)