My first question is, where is the extra oxygen needed coming from?
The molar mass of C is 12.011 g/mol.
The molar mass of O2 is 31.998 g/mol.
The molar mass of CO2 is 44.0095 g/mol. (C=27.3%, O2 = 72.7%)
The mole fraction of CO2 in Earth’s atmosphere is 417.
The mole fraction of O2 in Earth’s atmosphere is 209,460.
72.7% of 417 = 303.159
209,460 + 303.159 = 209,763.159
This means that if all the CO2 in the atmosphere was converted into O2, the percentage of O2 in the air would increase from 20.946% to 20.976%.
Oxygen toxicity occurs when exposed to oxygen at partial pressures greater than normal atmospheric pressure.
“Pulmonary toxicity occurs only with exposure to partial pressures of oxygen greater than 0.5 bar (50 kPa), corresponding to an oxygen fraction of 50% at normal atmospheric pressure. The earliest signs of pulmonary toxicity begin with evidence of tracheobronchitis, or inflammation of the upper airways, after an asymptomatic period between 4 and 22 hours at greater than 95% oxygen”
“At partial pressures of oxygen of 2 to 3 bar (200 to 300 kPa)—100% oxygen at 2 to 3 times atmospheric pressure—these symptoms may begin as early as 3 hours into exposure to oxygen.”
Sea-level air has a partial pressure of oxygen at 0.21 bar and oxygen toxicity does not occur below 0.3 bar. The thickness of the atmosphere would need to be increased to cause oxygen toxicity in the surface population.
Might I suggest either moving the humans in your story to another planet, one with higher O2 concentrations and a thicker atmosphere, or changing the initial threat to something like hydrogen sulfide. Perhaps hydrogen sulfide eruptions from the oceans caused humans to move into a domed city. The risk of HS getting through the dome worried the population, so they over-pressurized the dome with excess O2. Any leaks would force air out of the dome due to the positive pressure.
An increase in oxygen can have many positive impacts on the human body. More energy, less fatigue, and improved infection resistance to name a few. There are negative impacts as well, aside from oxygen toxicity, but these are less well documented. Oxygen therapy has been noted to increase the chances of death in certain cases.
Let’s take our hypothetical future. The humans are living in the domes to protect against the HS gas outside which has killed off most of the rest of the surface life. Initially, the increased oxygen had a beneficial impact on the inhabitants. They knew about the risk of oxygen toxicity and kept the levels at what they imagine are safe concentrations. Changes begin in two primary populations with two subcategories. The first were those with weak lungs who operated at a normal level due to the increased oxygen and thus were able to thrive. The second were those with greater lung capacity who were negatively impacted due to the excess oxygen. Within each population, you had those who were energetic/active and those who were lethargic/sedentary. An active person who had great lung function/capacity was more likely to experience negative health effects and die as a result. Due to a lack of resources, those who are unable to survive without constant medical attention would likely be allowed to die. This includes infants who are already more sensitive to oxygen levels. These deaths might not be attributed to the increased oxygen levels, but to other factors.
Eventually, we are left with three population genotypes, the sedentary weak lungs, the sedentary strong lungs, and the active weak lungs. People with similar interests and lifestyles tend to group together. The sedentary populations would mix, and their offspring would become sedentary with average lung strength. Occasionally there would be those who were active with normal/strong lungs, but that would work against them. Smaller/weaker lungs would become a beneficial trait.
There comes a time when the active population determines they want to increase the oxygen percentages. The increased concentration of oxygen would not only give them more energy, but it might also get their “lazy” neighbors off their butts and moving around. So again, we have a change to the environment which impacts the survivability of the humans. The cycle continues and the changes become more drastic.
Another thing to consider is that taller people need stronger hearts, pump more blood through their bodies, need more food, and take up more space. If food rationing is implemented, those with smaller frames would likely do better than the alternative. Having a limit on food while also being exposed to changes in the environment is a recipe for population adaptation. The population could easily skew towards smaller more compact bodies.
In the end, we could end up with a population that is generally sedentary but capable of short bursts of quick/energetic movement. They would be unable to survive outside the dome even if the hydrogen sulfide gas was not an issue due to the thin atmosphere and their weak lungs. At least, they could not survive on the surface. They could potentially colonize the ocean and live in pressurized underwater habitats. Depending on why you need the humans to change, you can invent whatever justification needed to force them to change. Perhaps they released a retrovirus which caused rapid mutation in the population and only those with beneficial (or at least not harmful) mutations survived. Maybe a disease killed off most of the population and they had to genetically engineer a new generation to preserve humanity. There could have been unintended consequences in subsequent generations. The dome is the limit.