I recently found that each degree of warming increases water vapour by 7% and I've also learned that the atmosphere contains (on average) between 1-5% water vapour when saturated, this confused me a bit, and I wondered if this could affect the oxygen concentrations in big cities or maybe the effect is too small.
The atmospheric concentration of oxygen is 20.8 percent, which is a significantly large proportion of the atmosphere.
The burning of fossil fuels is more of problem for oxygen depletion than the amount of water vapor in the atmosphere. There is no risk of hypoxia due to increases in atmospheric water vapor. What risk there is is of increased humidity and temperature discomfort resulting from that.