Question

If we take more oxygen in our body the water in our body decreases. Why?

Answer 1

I assume you are not a fish so directly it does not affect the oxygen intake into the body.

........going forward: Humans breathe oxygen in to their lungs. Where the oxygen is absorbed into the body by blood and then circulated all around the body.

Now the viscosity (thickness or thinness) of blood is affected by your water intake .....so lower the water content in the body .....the blood become less effective in doing its duty/function ...thus Water does affect the intake of Oxygen in the body.

Optimum amount of water in the body is necessary for the body to function healthily. Balance is required......more is not good & ..less is not good too. Optimum amount is what is required.

How much is required for you depends on many factors.....the climate, what level activity you are doing, Health levels etc. that's a different topic so not going into much detail here.