For the same pressure/temperature, a mix of gases with water in it will have less of the other ingredients than if there were no water. At 298 K/77 F, saturated water vapor pressure (corresponding to 100% humidity) is
3.17 kPa, which would imply that 3.1% of oxygen (and nitrogen) is missing, compared to 0% humidity. For comparison, at 3050 m/10000 ft
31% of oxygen (by mass) is missing compared to the sea level. The decrease in oxygent content due to the extra water at 77F/100% is approximately equal to the decrease due to altitude at 1000 ft.