Before the Great Oxygenation Event, where was the oxygen? Today, the oxygen in the atmosphere is about 1018 kg.
The estimated carbon biomass of the earth is about 4×1015. Less than 0.5%.

Estimated fossil fuels are around 5 times as much as existing carbon biomass. If these estimates are appropriate, then probably before the oxygen was released it was not locked up in carbon compounds.

Oxygen could have been extracted from inorganic stuff and cycled through the biomass. 

Which reduced minerals were left behind? Not silicates and carbonates. Little magnetite crystals? 

Maybe there was more SiO4 before (things like olivine), and living things tended to produce silicates with less oxygen (things like kaolin)? There isn't a whole lot of silica dissolved in seawater at any one time, but there's a whole lot of silica available and it could add up eventually. Silicates with more oxygen weathered away, and silicates with less oxygen were deposited?

Maybe there's so much free oxygen today because some H2 got produced, and it rose right out of the atmosphere and got swept away. So there's less water and more oxygen than there used to be.

Maybe the biomass estimates, and the fossil fuel estimates, don't actually tell us about the carbon, and there is lots of reduced carbon elsewhere.

I'm speculating, does anybody have good answers?
the great oxygenation event was caused by the evolution of photosynthesis, photosynthesis turns CO2 and water in to sugar and oxygen. so the oxygen came from CO2 and water. 

keep in mind CO2 levels were many many times higher (~20X) than today. Hydrogen was not released it was combined with carbon and oxygen to make sugars