How was oxygen on earth produced?

"THE EARLY ATMOSPHERE

The present composition of the atmosphere is:		21%	OXYGEN
		78%	NITROGEN
		0.04%	CARBON DIOXIDE
		~0.9%	ARGON

The atmosphere wasn't like this when the Earth was created over 4½ billion years ago.
THE FIRST BILLION YEARS
The Earth's surface was originally molten, as it cooled the volcanoes belched out massive amounts of CARBON DIOXIDE, STEAM, AMMONIA and METHANE. There was NO OXYGEN. The STEAM condensed to form water which then produced shallow seas.
Evidence points to bacteria flourishing 3.8 billion years ago so this means that life got under way about 700 million years after the Earth was created. Such early forms of life existed in the shallow oceans close to thermal vents, these vents were a source of heat and minerals.
THE NEXT BILLION YEARS
These primitive life forms then took the next evolutionary step and started to PHOTOSYNTHESISE (using sunlight to convert carbon dioxide and water to food energy and oxygen). This was an important turning point in Earth history because the carbon dioxide in the atmosphere was being converted to oxygen.

These green plants went on producing oxygen (and removing the CO2).

Most of the carbon from the carbon dioxide in the air became locked up in sedimentary rocks as carbonates and fossil fuels. Carbon dioxide also dissolved into the oceans.

The ammonia and methane in the atmosphere reacted with the oxygen.

Nitrogen gas was released, partly from the reaction between ammonia and oxygen, but mainly from living organisms such as denitrifying bacteria. (remember that nitrogen is a very unreactive gas and it has built up slowly).

THE LAST 2½ BILLION YEARS OR SO
As soon as the oxygen was produced by photosynthesis it was taken out again by reacting with other elements (such as iron).This continued until about 2.1 billion years ago when the concentration of oxygen increased markedly. As oxygen levels built up and then . . . . . .
The ozone layer was formed which started to filter out harmful ultraviolet rays. This allowed the evolution of new living organisms in the shallow seas."