Are oxygen levels falling?

Oxygen is elemental and non-radioactive; it would take far more energy than the burning of hydrocarbons to destroy the oxygen. The oxygen atoms have simply attached themselves to other molecules - they are still there.

On the other hand, if CO2 has increased to 380ppm from 200ppm in the past 250 years, then it stands to reason that something(s) else has decreased by 180ppm (you can only have a total of 1,000,000 parts per million!). As the major constituent of the atmosphere (N2) is stable and largely non-reactive, then it probably isn't N2 that has decreased.

Furthermore, the main argument behind the AGW theory is that the burning (reacting with oxygen) of fossil fuels releases carbon into the air as carbon diOXIDE!

So, yes, O2 in the atmosphere is combining with carbon (C) released from fossil fuels (and other sources) to become CO2 - oxygen levels are falling, more or less in line with the increase in CO2 concentrations.

Edit to CrazyCon:
This argument - that higher CO2 levels are good for plants - is often used as if it is the answer to AGW. This is totally wrong:

1 - There is some evidence to show that some plants in some areas are doing better due to higher CO2 levels.
2 - CO2 levels continue to rise despite this.

The obvious explanation for this is that, although some plants absorb more CO2, plants are obviously not capable of absorbing all the excess hence cannot provide a solution to our dumping of megatonnes of carbon into the atmosphere.

Humans have added a lot of carbob to a once-stable equilibrium. If all that carbon had remained in the atmosphere, CO2 levels would be have increased from 200ppm to around 500ppm instead of the actual observed amount of 380ppm.
The 'missing' 120ppm (500-380) shows natures coping mechanisms in work - absorption by oceans, increased plant growth etc.
The 180ppm remaining in excess (380-200) shows the overflow - what nature cannot absorb and thus remains in the atmosphere.

Worryingly, this tends to suggest that natural coping mechanisms (including extra plant growth) is at 100% capacity - any further emissions will accumulate in the atmosphere without any mitigation.

This reasoning will lead sceptics to stock up on paper towels when flooding in their area becomes common - "but paper towels absorb water! I'll be fine..."

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The oxygen (O) is not destroyed when something is burned, it is the attaching of the oxygen to the carbon (C) as CO2 or CO (or to hydrogen, (H), which makes water, H2O) that releases the energy. so you still have the oxygen but it is stuck inside a bigger molecule, until released by, for example, plants (which want the carbon as a building material), which use sunlight to split the carbon away from the oxygen again and release the oxygen back into the air.
the only way we loose oxygen forever is the small amount that escapes into space, but we dont need to worry about that for another few billion years.

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The hysteria over oxygen depletion is so desperate and ill-informed it's amusing. The oxygen content in our atmosphere remains constant at around 21%. The CO2 level, even though it's increasing, still hovers around .04% of the atmosphere. So we may have displaced some oxygen with CO2 but in such a tiny amount it would be impossible to measure it with any certainty. Keep in mind, plants love CO2 and they produce oxygen so more CO2 is good in many ways, and warming has always been good for humanity as well. Read about the Bronze Age warming, the Roman Warming period, and the Medieval Warming period, all were times when temp was higher than it is today and harvests were bountiful and humans flourished.

There is no evidence at all that oxygen levels have fallen or will due to any human activity.

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Oxygen is conserved. Consider what would happen to the oxygen content of the atmosphere if all of the biomass burned.

Atmosphere 5.1441 × 10^18 kg
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. D11, 23,079–23,088, 1994

The mass of O2 in the atmosphere is 20.95% of the total mass or 3.23 x10^17 kg.

Biomass 1.9 x 10^15 kg
Whittaker, R. H.; Likens, G. E. (1975). "The Biosphere and Man", in Leith, H & Whittaker, R H: Primary Productivity of the Biosphere. Springer-Verlag, 305-328.
I think this estimate is low, but the order of magnitude should be right. Taking the most abundant biopolymer cellulose as a typical composition for organic material (C6H12O6)n. On a per C basis, combustion consumes 1 molecule of O2 and yields 1 molecule of CO2 and 1 molecule of H2O. On a molar basis, 30 g of organic material react with 32 g of O2. Hence, if all of the biomass reacted with O2, 2.0 x 10^15 kg of O2 would be consumed. This represents just 0.63% of the mass of O2 in the atmosphere, so the gross composition of the atmosphere would not change much. However, the atmospheric CO2 concentration would increase by a factor of about 4.

Excerpts from:

http://answers.yahoo.com/question/index?qid=20080830120619AA0x7px