Think this is wrong. The discarded oxygen during photosynthesis comes only from splitting CO2.
https://en.wikipedia.org/wiki/Cyanobacteria#Metabolism
"In general, photosynthesis in cyanobacteria uses water as an electron donor and produces oxygen as a byproduct, though some may also use hydrogen sulfide[77] a process which occurs among other photosynthetic bacteria such as the purple sulfur bacteria."
* I found it by searching for "oxygen"
Contrary to grandparent, in oxygenic photosynthesis, the oxygen does come from splitting water. The resulting hydride used to "charge" an electron carrier: https://en.wikipedia.org/wiki/Photodissociation#Photolysis_i...
The CO2 gets fixed into carbohydrates. Some water is generated in that process, but no molecular oxygen.