Follow in the footsteps of the first alchemist, Ernest Rutherford.
Rutherford changed Nitrogen in Oxygen by bombarding nitrogen with Alpha particles. However, you will need a truly monumental amount of alpha particles to make a measurable difference in Oxygen. On the plus side, you won't miss the nitrogen you convert. Since this is really difficult perhaps you might like other solutions ...
You could take oxygen from the Sun. Oxygen is about 0.78% of the sun, but the sun is so massive, that the sun's oxygen outmasses the earth. Still very, very difficult.
Siphon off carbon dioxide and/or sulphur dioxide from Venus and leave the non-oxygen atoms behind. You need to really improve space travel, but this is more practical.
Refine a large amount of oxygen bound molecules in the crust. Iron, silicon, or Aluminum (and others) are desirable in the elemental form, just strip out the oxygen from a small fraction of iron in the crust and you will have all the oxygen you need. This is almost easy in comparison -- you need lots of energy, and mining (in some form) but we already do this on a daily basis -- just not nearly enough to measure the result.
One problem that all approaches face is that compared to us puny humans, there is a lot of atmosphere. We have been pumping gigatonnes of CO2 into the atmosphere every year for quite a while now. By doing so, we have managed to raise CO2 levels up to about 400 parts per million. I know that carbon is captured by plants, oceans, etc. but estimates are around 2000 gigatonnes since 1750, of these 44% are still in the atmosphere, so about 880 gigatonnes were added to the atmosphere raise CO to current levels.
Adding 5% oxygen is inherently a much more difficult task (you can't just set carbon on fire) and a much greater larger mass change required to satisfy the requirements. I.e., change of about 125 ppm for CO2 vs. about 10000 ppm for O2, i.e., 80 times as much.
Note that none of these processes are natural, nor can any natural process do what is asked for. Significantly changing the oxygen mass requires engineering of some form. Natural processes do not supercharge the oxygen levels, or they would already have done so -- free oxygen is not the natural state of a planet, there will always be more iron, silicon, etc. to use all of the free oxygen in an earth-like planet. You have to add free oxygen after the crust and its chemical composition have stabilized. Life is generally considered the only way that this happens in real life.