Can you back this up with any data at all? You do know that plants breathe CO2 and exhale oxygen, right?
Of note, the current concentrations are 209 460 ppm of O2 compared with around 380 ppm of CO2. So, it's currently a non issue. But again finite systems are finite.
PS: I only skimmed this as the first Google link: http://www.i-sis.org.uk/O2DroppingFasterThanCO2Rising.php. No idea if it's credable, so feel free to search.
I certainly agree it's "currently" a non-issue; but I would add that given the numbers in the article it's going to continue to be a non-issue for the foreseeable future. The key fact from the article is that the rates of change of O2 and CO2 are of the same order of magnitude: a few ppm/year. That change is a percent or so of total CO2 concentration, but only a thousandth of a percent or so (1 part in 10^5) of total O2 concentration. So it would take a thousand years at current rates for the O2 concentration to decrease by 1 percent of its current value (which is still negligible in human terms).
Plus, that assumes a linear decrease over a thousand years, which is highly implausible. The article notes that we have only been measuring atmospheric O2 concentrations for a couple of decades. No meaningful conclusions can be drawn from that about what will happen to O2 concentrations over a thousand years. The best current model we have is the simple and obvious one that O2 concentrations in Earth's atmosphere are maintained at appropriate levels for oxygen-breathing animals as part of the cycle of plant and animal respiration, which has been going on for hundreds of millions of years, including periods when CO2 concentrations were much higher than they are now.
Hypothetically, you can slightly adjust the comet's orbit and save ~10% of the global population or do nothing. You have 30 seconds to decide and failure to act within 30 seconds kills everyone. The longer you wait the more people die.
PS: Sure, your unlikely to be in that situation. But, the earth is a finite system and physics does not care. Or as someone on Easter Island learned you can cut down the last tree.
Abstractly, the environment is clearly worth more than every other thing in the world combined. EX: If 6.9 billion people needed to die to maintain a breathable atmosphere that would be a simple choice. How to messure lower levels of harm is what's subjective.
PS: Consider burning fossil fuels has directly lowered the amount of free oxegen in the atmosphere. There is not enough fossils fuel know to exist for this to be a real problem. But in a world where there was it would be a much more direct survival issue.