Question

how oxygen is exist on earth

Answer 1

it is exist due to transportation of plant and plant take co2 and provide O2

Answer 2

Joshua Engel is correct that all oxygen was formed in stars, most of it in the later stages of stellar evolution. Oxygen and other heavy elements on Earth are evidence that Earth formed from the stellar debris of a supernova as well as interstellar gases. Only the lightest elements (hydrogen, helium, lithium and perhaps traces of slightly heavier elements) could be formed in the Big Bang itself. Oxygen itself comes from the late stages in the life of a star, when the star fuses helium to form even heavier elements. But elements that are heavier than iron could only be formed in a supernova. Generations of massive, but short-lived stars may have contributed to the early Earth's supply of oxygen and other heavy elements.

I think the questioner is really asking, where did the free oxygen (O2) in Earth's atmosphere come from? Oxygen was a part of Earth from the earliest times, but at first almost all oxygen on the planet was chemically bound in both organic and inorganic compounds, as well as in abundant molecules such as water and carbon dioxide.

Oxygen is exceedingly reactive, and iron is one of the most common elements in the Earth's crust. In the early history of Earth, most iron at or near the surface of the Earth was not yet oxidized, so free oxygen was quickly absorbed into the Earth's crust. The lack of free oxygen in the atmosphere probably prevented more advanced organisms from developing. The earliest life forms were based on anaerobic metabolisms, although aerobic metabolism offers great advantages. Since free oxygen was quickly absorbed, it was impossible for organisms based on aerobic metabolism to develop.