When you're dealing with one constituent of a gas mixture, it's partial pressure that counts, not the pressure of the whole mixture. If the proportion of oxygen

When you're dealing with one constituent of a gas mixture, it's partial pressure that counts, not the pressure of the whole mixture. If the proportion of oxygen in the atmosphere were to increase without a change in overall atmospheric pressure, the partial pressure of oxygen would increase. Another way of saying this is, you would have more oxygen molecules in the same volume of space. The total number of all molecules per unit of volume would be the same, but more of those molecules would be oxygen. More oxygen molecules per unit of volume = higher pressure.