I am not sure what you mean by "without oxygen". Oxygen gets into the water from air. Having some agitation of the water such as from an airstone or HIB filter can help preventing the water from becoming so stagnant the fish end up air gulping at the surface. But I don't know that plants really contribute significantly to O2 saturation, I suppose it is possible.

I am also confused with the term "without oxygen." Actually water is two parts hydrogen and one part oxygen. There is no water without oxygen. If you mean to ask whether you can maintain oxygen level in the tank by having too many plants, I think it is possible.

Hkb

Well what I know about fishes is that they need oxygen to survive, so even with having lots of plants in the tank and it still lacked enough oxygen to sustain all the fishes in the tank, they are definitely not going to survive. Keep the water fresh and neat, and change it from time to time, it's the only way to keeping the fishes safe.

I think they can but it's not a good idea, the problem with surviving without air pumps is when the plants is not producing enough oxygen. Also there's a with a tank without pumps and filters regarding toxicity.