Earth has regions of high/low oxygen. As example, climb a mountain. If I wanted to make a planet with more exaggerated zones, I would think of it in terms along those lines.

So it would become more a mater of atmospheric density rather than the concentration of O2 being different in different places. Maybe the low-lands, in canyons and what-not would have enough O2 because the air density is high enough but the higher one goes the less O2 is to be had because, again, the air density changes at higher elevations.

As time passes and the terraforming goes on, more atmosphere is made and zones change accordingly.

Earth has regions of high/low oxygen. As an example, climb a mountain. If I wanted to make a planet with more exaggerated zones, I would think of it in terms of those lines.

So it would become more a matter of atmospheric density rather than the concentration of ${O_2}$ being different in different places. Maybe the low-lands, in canyons and what-not would have enough ${O_2}$ because the air density is high enough but the higher one goes the less ${O_2}$ is to be had because, again, the air density changes at higher elevations.

As time passes and the terraforming goes on, more atmosphere is made and zones change accordingly.