What I have learned from this is that things were very different before the Great Oxygenation Event; more than just the same elements rearranged.
If all of the free oxygen came from water (and the hydrogen escaped to outer space), that isn't very different. That's around 10^18 liters of water lost from the oceans. The surface of the earth is about 5x10^14 square meters, one liter covers 10^-3 square meters, so sea level would have been somewhere vaguely around 2 meters higher. There's a whole lot of water in the world.
People say the atmosphere used to have 20% CO2 and that's where the oxygen came from. I didn't believe it because there is probably not that much reduced carbon in the world today. But it's possible that as the carbon was reduced, it was deposited on the sea floor and then it got subducted deep underground. It may still be there, waiting to get out.
Maybe there were other oxidized compounds that got reduced. The obvious candidates are nitrates and sulfates.
Today, bacteria mostly reduce nitrates when there is not enough free oxygen. In a series of steps they reduce it all the way to N2, getting energy at each step.
Other bacteria convert N2 to ammonia, using considerable energy. Often the ones that do this are photosynthetic. They have N2 and not enough nitrogen compounds to meet their needs, so they make what they need. O2 damages the enzyme.
Possibly there used to be a lot more oxidized nitrogen dissolved in the ocean, and the balance shifted, resulting in both increased N2 and increased O2.
Maybe there used to be a whole lot more sulfur in the ocean, in the form of sulfates etc. Most of the sulfur got reduced and deposited in the form of elemental sulfur or iron sulfide etc. Then it got subducted away, removing the evidence.
What was the pH of the ancient ocean? Various organic compounds can buffer pH differently according to concentration. H2CO2 (formic acid) acts different from H2 + CO2. Did the ocean used to be more acid? I don't know, and there were a lot of different buffers. Maybe H2CO3 (carbonic acid) was always the most important, if there was much more CO2 dissolved in the oceans than any other organic acid.
Just as this could happen for sulfur or iron, it could happen with any other good electron acceptor that happened to be dissolved in the ocean. Precipitate it out, and then subduction removes most of the evidence. But only things that had oxygen and then lost it, would contribute to free O2.
It was very different, and I can't even assume that the amounts of the various elements present were like today.
If it isn't clear what the pH of seawater was back then, I can't even be sure what the maximum amount of dissolved stuff would be. When I tried to guess at the maximum amount of dissolved iron, it turns out it varies with pH, with the amount of other dissolved chemicals, strongly with the amount of organic carbon compounds (because living things in today's ocean can't get enough iron and they scavenge it aggressively), etc. But that's today's ocean, where dissolved iron is quickly oxidized.
I expect there's room for a whole lot of uncertainty about all that. But experts can still know some things about it.