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once the cells have acquired as much oxgen as they need, they are in a state of 'saturation'. The concept of too much oxygen is not an issue - unless we look at human phisogamy. Excess oxygen will be absorbed into tissues. ns subsequent reactions will occur within the body. None favorable to the puny human. At least with nitrogen you get a good buzz.

Active transport involves a process that although somewhat chemically simple to describe involves a large number of interrelated processes. sorry to say this, but lookup Krebs Cycle, to see how O2 and CO2 are processed.

How about advantigous roots and cypress knees. The roots have evolved to get their oxygen.

You can't get oxygen underwater, hence the development of these devices. These are sure signs that you are in a wetland.

more the oxygen more the respiration more ATP more active transport and in the roots its the active uptake of ions like nitrates which requires oxygen for respiration and active transport