where would the oxygen come from?
Please tell me you're kidding. Did you sleep through plant biology in high school?
You seem to be deaf to that whooshing sound above your head. But, hey .. let's parse it it out anyway.
In photosynthesis we basically have CO2 + 2H2O + photons => [CH2O] + O2 + H2O
Do you notice that there is the same number of O's on both sides of the equation? That means that no O was created in the process, which means your derision about the OP is unfounded as photosynthesis does not create O2, it merely frees O2. EG all the O we need and use already exists in the world.
Thus the OP was correct in stating that levels of O can only go down and not up, as the only place you can normally create new O is in the center of a star. But O could potentially escape into space (as I believe that He does)
The trap you fell for was that TFA is talking about the shifting of O trapped/released from various sources within the Earth's environment as a closed system, whereas the OP was implying O exfiltrating from the Earth. So Apples and Oranges.
All in all it was a pretty good troll. Factually correct, with a huge dose of easily over-looked assumption.