Being humans on Earth, we have an implicit understanding that oxygen is a pretty important thing for our survival.  Oxygen is a necessary part of how our bodies function, taking in breathable air, cycling oxygen through our internal systems, and exhaling carbon dioxide as the waste of that process.  Some of us may even be privy to the fact that plants help play a role in ensuring we're not just breathing out our ultimate demise by taking in our carbon dioxide waste for their own processes and expelling oxygen for us to breathe once more.

This is all a great way to understand that our very livelihood is possible thanks to a well-balanced system.  This system, the oxygen cycle, does get a bit more complex than simply our relationship with terrestrial plants, however.  In fact, what we described above only makes up about half of how oxygen moves around the Earth, and our atmospheric oxygen only makes up about 1% of the oxygen available on the planet.  I know, that was a bit of a surprise to me too!  So let's get started, there's a bit of ground to cover here.
Let's dive in!

The Importance of Oxygen

Humans, like most other complex animal organisms on Earth, evolved to use oxygen to perform cellular respiration.  This is the result of our cells generating the energy we need to, well, do anything.  How this works varies from creature to creature.  For humans and other mammals, we have our respiratory and circulatory systems to do two things for us: take in a ton of oxygen and move it to where it needs to be, respectively.  It's this same system that allows us to expel carbon dioxide, the output of our cellular respiration.

​This concept of getting oxygen to our cells is ubiquitous across large organisms.  In our simple-celled organism brethren, it's easy enough for them to diffuse oxygen right through their "skin," but the bigger the organism, the more cells we have, which requires systems to move oxygen to those cells before they suffocate.  It's kind of awesome to see how not only the balance of our planet's systems play into life on Earth, but even how that very life's existence is a balancing act of internal systems.
Most heterotrophs on the planet require oxygen to create their energy

The systems involved differ from creature to creature, but have the same result
Suffice to say, how life on this planet evolved to energize itself is the primary driver in the importance of oxygen today.  What's curious, though, is that getting to that point was not how the Earth started.  On a planet so entrenched in hydrogen, how that planet managed to shift to an oxygenated atmosphere is an anomaly in our solar system.
99% of the Earth's oxygen is locked in the planet's crust and mantle!

Where is (and was) Oxygen?

Oxygen, historically, was primarily locked in the Earth's lithosphere.  As this geologic oxygen cycled into other reservoirs, life on the planet's ocean surface did use this to support themselves.  These were small oxygen oases where simple organisms would use the oxygen much how we would see today with photosynthesis.  After this point, a phenomenon scholars call the Great Oxidation Event occurred which proliferated the use of oxygen in these organisms, spreading across the surface ocean, bringing an equilibrium between the ocean's oxygen and atmospheric oxygen.
That isn't to say oxygen burst from the Earth's lithosphere into the atmosphere.  Even to modern day, 99% of the Earth's oxygen is held within the planet's geology with weathering being the way it eventually unlocks into other reservoirs.  That last 1% is distributed among the atmosphere, biosphere, and hydrosphere.  That 1%, however, was enough to cause an massive shift in the life that took hold on Earth.  Another name this event goes by is the Oxygen Crisis with reference to the mass extinction of life that couldn't handle this shift in the Earth's atmosphere.
On the modern Earth, organisms interact primarily with that 1% of oxygen in the atmosphere, hydrosphere, and the biosphere of the planet.  While that may sound like a thin layer of oxygen to work with, this is where the most active parts of the oxygen cycle take place, and what keeps this precious resource circulating to sustain life as we know it across the Earth!

How Oxygen Moves

Now that we have an idea of where oxygen can be found, how does it make it from place to place?  As described earlier, one of the key ways this is done that most of us may be aware of is photosynthesis.  This is the process plants use to pull carbon dioxide out of the air using the sun's energy to pull carbon out of that molecule for it to grow while expelling oxygen from the molecule as waste.  This process is key to how terrestrial life has enough oxygen in the atmosphere to survive.  Complex organisms take in that oxygen and expel carbon dioxide in a balance that keeps both parties from running out of "breathable" atmosphere.
Oxygen is introduced to the atmosphere by autotrophs performing photosynthesis

​We can thanks our green plants on the land and phytoplankton in the sea for the help!
Did you know that only accounts for roughly half of the Earth's cycled oxygen though?  The other massive player in cycling oxygen is phytoplankton on the ocean's surface.  That's right. the same organisms that brought the hydrosphere and atmosphere into oxygenated equilibrium are still hard at work today!  These organisms function as the plants of the ocean, performing photosynthesis at the top-most area of the marine biome.  In what is known as the Earth's biological pump, these organisms cycle in carbon from the atmosphere, expelling oxygen back while cycling carbon, nitrogen, phosphorus, and silicon into the deeper areas of the ocean.
Oxygen can also cycle into the Earth by way of ultraviolet radiation freeing hydrogen and nitrogen into space, leaving free oxygen behind
One of the other primary ways oxygen cycles is by interactions with ultraviolet radiation and the outermost layer of the Earth's atmosphere.  It's here where the planet's water and nitrous oxide can split, causing the displaced hydrogen and nitrogen to exit into space, leaving the free oxygen in the atmosphere.  This is a very minor part of the oxygen cycle, but still a more active part than oxygen leaving the lithosphere, so worth mentioning.  It's also one of the few interactions where the planet interacts with space, trading out some elements for free oxygen in our atmospheric reservoir.  Neat!

Breaking a Balance

As with most of Earth's systems, humanity has generally taken the oxygen cycle for granted.  Growing up, I largely assumed that an oxygen-laden atmosphere was a given and that most of the Earth's oxygen was there.  As it turns out, the oxygen cycle, like other biogeochemical cycles is a fine-tuned, but ultimately fragile balancing act.  That we have an oxidized atmosphere is a stroke of luck that scholars still debate from aspects of its happening to this day.
That said, the biggest impact humanity has on the oxygen cycle is in maintenance of its balance.  We don't really do much that causes us to breathe more or less, though an over-populous planet could eventually make oxygen availability an issue, that's not an immediate concern for the planet.
What is a concern, however, is the balance of oxygen with other parts in our reservoirs.  As humans impact the carbon cycle by pushing more carbon dioxide into the air and the nitrogen cycle by dumping excess fertilizer chemical into the ocean, we face balance issues for our plants and phytoplankton.  We've seen how these nitrogen cycle imbalances can cause algal blooms ultimately result in anoxic (oxygen devoid) regions that either kill off or force creatures to migrate.  We've seen how carbon imbalances can cause phytoplankton to overproduce nutrients leading to ocean acidification.  These are all measurable changes to our equilibrium that have rippling effects.​
Oxygen is a balancing act where the biggest threat we pose is breaking the equilibrium we currently enjoy by disrupting other adjacent cycles
One thing worth mentioning, too, is that while that did focus on balances with the ocean, it is imperative to reflect on how we got that 1% of oxygen circulating for the planet to use.  That's right, our oceans.  While this is seemingly speculative, it's clear that the balances in the Earth's systems have effects that humanity doesn't yet fully grasp and we should do our parts to ensure our home (and the functions it offers us) stay healthy so life can continue to thrive as it has.
~ And, as always, don't forget to keep wondering ~
Sources
* Oxygen Cycle:
https://link.springer.com/chapter/10.1007%2F978-3-662-24940-6_5. Retrieved 2021-01-17.
* Respiratory Systems:
https://youtu.be/bHZsvBdUC2I. Retrieved 2021-01-17.
* The Great Oxidation Event:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1578726/. Retrieved 2021-01-17.
* The Biological Pump:
http://www.mathis-hain.net/resources/Sigman_and_Hain_2012_NatureEdu.pdf. Retrieved 2021-01-17.
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