Answer
I'll assume this means adding additional oxygen to the atmosphere.
Paper airplanes now fly farther. With all that extra air, the air pressure near the surface increases significantly. Gliders, parachutists, birds and paper-plane hobbyists enjoy greatly improved performance.
Everyone gets better gas mileage. Oxygen-enriched air improves engine performance by producing hotter reactions and reducing the proportion of nitrogen, which reduces heat transfer (Page on Energy.gov)
Higher elevation Biomes become inhabited by more vertebrates. Areas such as the high Himalayas or high Andes are no longer off-limits to animals without special adaptations to increase their levels of hemoglobin.
Insects of unusual size. Many insects rely on gaseous diffusion to for respiration, therefore the maximum body size depends on the proportion of oxygen gas in the atmosphere. Most bugs get bigger, some smaller (see Atmospheric oxygen level and the evolution of insect body size)
Everyone feels more alert, active, and happier. All that fresh oxygen improve our cognition, alertness, and physical performance. As a result, most athletic records would likely be broken by oxygen-enriched athletes.
We get sick less often. Neutrophils, soldiers of the immune system, destroy bacteria by using NADP oxidase to pump ions into, and disrupt, intruding cell's membranes causing rupture. More oxygen, more oxidase. (Page on Nih.gov)
We die younger. Free radicals (i.e. O2-) are thought to exacerbate the aging process through Oxidative stress, which interferes in numerous cellular processes: protein production, DNA replication, intercellular communication, and are also thought to contribute to MS, Alzheimers, Parkinsons, and a host of other ailments.
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