how oxygen come to our earth?

Oxygen is made inside stars. When a massive star dies and explodes in supernova, oxygen is blown into outer space and becomes part of a giant cloud of gas and dust from which new stars may form. This means that when planets forms around a new star, there is already some oxygen there, which can become part of there planet's atmosphere.

If you piece together all of the different answers that you got, you can begin to see the picture.

Oxygen is a highly reactive substance. It does not last long in nature before it gets bound up with other atoms. All of the atoms in and on the Earth (including all of the oxygen atoms) were made in stars that lived and died long before the Earth was formed. The oxygen on the primitive Earth was all bound up with other atoms. Some of it was in the form of water (H2O), and some of it was in the form of carbon dioxide (CO2). Green plants evolved the ability to use sunlight as an energy source, and they consume CO2 and H2O, and liberate oxygen gas (O2) in the process.

All of the free oxygen in Earth's atmosphere is made by green plants (most of it by algae in the world's oceans). Without the plants continually renewing the supply, it would all eventually become bound up once again with other substances.

The oxygen in early Earth is contributed by the algae as life starts to form.Now 50% of of the Earth's oxygen is produced by phytoplankton in the seas and the other 50% by the trees and plants on land.

some was here from the start in the form of water.

Stromatolites ----- Cyanobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis.

the theory is that the planet was carbon dioxide rich, and therefore plants evolved using co2 as their "breathing requirement", the bye product was oxygen, O2. This then became more abundant and other life evolved to use this as their source.

Some scientists describe three stages in the evolution of Earth’s atmosphere as it is today.

http://scijinks.jpl.nasa.gov/atmosphere-formation

Millions and millions of years ago, while the earth was in one piece, many earthquake and volcano erupted,split the land in many parts created gas, a blend of these gas created our oxygen

in canisters from the distant planet of Oxyklorod

They saw this planet 4.6 billion years ago and sent billions of canisters to help it grow into the planet it is today