Thin air here - but maybe not every day?

The barometric pressure is never, never actually 30.11"Hg in Denver. The actual pressure (which you say is 24.76, and I'll buy that) is converted to sea level equivalent.

There is a very good reason why this is done. It's not just higher elevation locations, e.g. Denver, where this occurs. It is standard practice for every location on land. Even places like Cleveland Ohio, which has an elevation of about 500 feet, have their barometric pressure reported in MSLP ("Mean Sea Level Pressure") equivalent.

For meteorologists to forecast weather, some "control" must be placed on the data. If pressures were reported and analyzed by absolute numbers, their data would be nearly meaningless. If one were to analyze surface pressures in absolute values, especially in regions where the elevation undulates a lot, such as CO, it would be impossible to ascertain where a surface low (trough/cyclone) or high pressure ridge existed. This would make weather forecasting nearly impossible, since knowing where surface lows and highs exist is crucial to forecasting the weather.

Even absolute pressure differences between Cleveland and Philadelphia would be unanalyzeable unless converted to MSLP because of the differences in elevations.

So yes, there is most certainly less oxygen in Denver than at sea level.

Just up here in Longmont where we're about 200' lower... Truth to tell, he was probably suffering from the heat and didn't know it. Afterall, we were up around 101F today, and we were in the high 90s before that. We like to give 'em the double whammy... not enough air and not enough humidity. if one doesn't get 'em, the other will! It's dry enough here that he might not have realized he was being "cooked". Figured he wasn't too sweaty, so how hot could it be? He probably made the same mistake that many who visit here make -- lack of fluids -- and was simply dehydrated. We always tell visitors that if they're feeling thirsty, they've probably waited too long. Good rule of thumb for flatlanders in climates like this -- pee clear once a day. Guarantees you won't dry up and blow away. The problem seems to be exacerbated by "altitude sickness" problems when our baro pressure is low. Not sure why. Sure does wipe them out on a hot dry day in the mountains, when we have them. Our baro here at the moment is the same 30.11 you're reporting down there - a bit more than the standard 29.92.

Barometric pressure and oxygen content are not necessarily directly related. Yes, higher elevation does indicate lower oxygen content, and a lower baseline pressure, but pressure changes that occur due to weather systems do not alter the oxygen content.