A new study from researchers at Princeton University and published in Science has confirmed that the earth is losing oxygen. According to their data, over the past 800,000 years, the earth has lost 0.07% of its oxygen. This isn’t enough to cause harm to life, though scientists are still wondering why this drop occurred.
To find these conclusions, researchers studied air bubbles trapped in ancient ice formations in Iceland and Antarctica. Within these bubbles, they estimated what atmospheric pressure was several hundred thousand years ago by measuring the ratio of oxygen to nitrogen in the bubbles.
Lead author Daniel Stolper, a geologist at Princeton University, stated:
“Every thousand years or so, all of the O2 [in Earth’s atmosphere] is turned into water and then back into O2, but there’s an ever so slight leak over time, in terms of extra production or consumption.” 
Geologists and researchers know that oxygen levels have changed in the past 800,000 years, but they are unable to tell if they have remained consistent. And while the idea of the earth losing oxygen might sound scary to some, the team emphasizes that this drop in oxygen is actually relatively small as it doesn’t harm any life on earth.
“To put it in perspective, the pressure in the atmosphere declines with elevation. A 0.7 percent decline in the atmospheric pressure of oxygen occurs at about 100 meters (330 feet) above sea level — that is, about the 30th floor of a tall building.” 
Although scientists remain unsure of why the oxygen levels have dropped over the past several thousand years, there remain 2 major hypotheses.
The first theory is that erosion rates have increased over time. This is due to the growth of glaciers, and glaciers grind rock, which contributes to these erosion rates.
The second theory is that the ocean has cooled over the past 15 million years before humans began burning fossil fuel. The ocean can therefore store more oxygen in the water at colder temperatures, and microbes that depend on oxygen who live in the ocean are able to consume it at a faster rate.
One result of the study which also perplexed scientists was that although oxygen levels had dropped, carbon dioxide levels have not risen (except in the last 150 years or so). One theory is that of a “silicate weathering thermostat,” which states that as carbon dioxide builds up, volcanic rock erodes and are taken out to sea, where they remain trapped.
 Live Science