Last time there was a thread about this I found a U.S. Department of Transportation (I think) study on nitrogen on tires. My recollection is that it found that the nitrogen concentration in air-inflated tires increased slightly over time. But I don't think the study concluded whether this was due to oxygen diffusing from the tires more rapidly, or due to it's being "consumed" in oxidizing the tire walls.
In any event, the change in concentration was quite slight, meaning that if the oxygen was diffusing faster, it wasn't by much. And when you consider that air is only 20% oxygen to start with, a slight decrease in the diffusion rate for that 20% isn't going to make much difference in tire pressure.