I believe oxygen concentration [breathable gases] decreases with humidity because the very same volume is filled with “gasified water” and there is less space for “air” molecules, BUT volume density of the gas you breathe increases because in the very same unit there are heavier molecules than before (moisture vs. gas). Oxygen / nitrogen and other atmosphere gases rise to give place to heavier H20 molecules. That way the atmosphere becomes more viscosius and it requires bigger forces to move arround in it.
Am I wrong?