Where on Earth does the atmosphere have the most $O_2$ concentration?
You may find bigger O2 levels at the upper layer of rich oceans -biologically talking-. There is Henry's Law, an equilibrium between gas pressures/concentrations of the ocean and the atmosphere, so you might find it at the contact between an ocean bacterial rich system and the atmosphere, but maybe on arboreal forests at light.

I have no data about O2 levels at both suggestions, atmosphere/ocean contact at a rich ocean system &amp; a very rich arboreal forest on hot season. Just a caution for a possibly answer from a botanic. There may be maps about O2 levels at ocean surface but that at NASA and so and I am a student with no access to Science papers Muze and don't know how to get them. For arboreal system they can give you absolute maximum O2 levels registered, and then comparison.

The maximum level will happen at a certain time, and even I think satellites can measure gas levels the layer to search is about 1mm or so. maybe doing a comparison between highest O2 levels measured at the interface of a rich estuary or so and a tropical forest coming from local studies we find a non exactly answer but giving the environment where to find it and an approximate concentration.

There are those ozone/O3 layers with a very high concentration at industry zones,but i guess you ask for O2 gas. There are also hyperbolic conditions,eg at a Erlenmeyer flask of a lab, but I guess you are asking for levels on Earth's Atmosphere sensu stricto at a relative big atmosphere's surface/volume. I think others will appreciate more the question if you write eg "higher O2 concentration measured on an Earth's Atmosphere area despite labs or natural micro-layers" or something like that if that's you want to mean. Also sensu stricto more oxygen no matches with saturation (saturation is function of Temperature and Pressure).