Oxygen reached 35% during the Carboniferous. This lead to lifeforms taking advantage (e.g. giant invertebrates outside the seas) as well as rainforest wildfires as the oxygen levels significantly increased flammability of flammable materials (most of our coal dates back to these days, where rainforests would grow quickly, burn down to 10~20% charcoal by volume, and that charcoal would fossilise over time).
Somewhat oddly, very few things are hypergolic with oxygen, even LOX (which is as full of oxygen as you can get) or 100% oxygen atmosphere. Fuel will readily ignite, but it needs an ignition source of some sort.
This is in contrast with fluorine-based oxidants like ClF3, which is hypergolic with sand, asbestos, or water.
Speaking of things that 'splode, there's a lovely article that makes the rounds here every so often by Dr. Derek Lowe on the onomatopoeically named FOOF (dioxygen diflouride), one of his "Things I won't work with."
Though ozone is as toxic as fluorine and tend to spontaneously explode, according to Ignition! it can be made safer by mixing it with LOX but to reach safe use levels you apparently need to get down to ~25% O3, at which point it's just a harder to handle LOX.
It seems (I didn't remember this passage) folks also tried out mixing ozone with fluoride, which strikes me as a very Cave Johnson idea.
It goes like this: early earth was subjected to solar radiations which hit our liquid water, splitting H2O into separate oxygen and hydrogen elements. The very light hydrogen atoms (as ions, or maybe H2?) were escaping into space while the oxygen was trapped by oxydizing rocks.
With the rise of cyanobacteria produced O2, the rocks became fully oxydized and O2 started to build up in the air: the split hydrogen would be more likely to recombine with atmospheric O2, keeping them on earth, and the ozone (O3) layer started to shield the water from these radiations in the first place, thus stopping the process and safeguarding our oceans.
 https://www.amazon.com/dp/0198607830 (very cool read mixing early earth history, the rise of life, geology, biochemistry and phylogenetics + Dr Lane is an excellent writer. Published in 2002, he has many more good updated articles about it.)
Cyanobacteria turned 20% of the atmosphere into very reactive O2.
I like to point that out when people say that we are destroying our planet. Our planet will be fine with or without us, we are just hoping that it will be with us.
Humans should definitely make a big deal about their own survival.
I like that, however I think hoping and doing something about it are two different things.
It covers the great oxidation event in some detail, and covers all of the super-early single cellular and multicellular life history in great detail.
So oxygen is a sign of life. Can we measure oxygen's spectral line from any exoplanets? Would enough O2 suggest our kind of life is more likely there? If we could survey exoplanet atmospheres in our neighborhood for oxygen would that tell us anything about the distribution of life?
It seems plausible that on many planets you have life evolving but because it's slow to develop photosynthesis the solar wind strips away all the hydrogen before the production of oxygen creates a protective ozone layer. Maybe that even happened on Mars, though with the lack of magnetic field there and shallow gravity well it was going to lose its hydrogen eventually even with an ozone layer.
Looks like determining abiotic oxygen situations in the "goldilocks zone" versus potentially biotic or life-supporting is an ongoing field of exoplanetary research: