Despite the common belief that vegetation is responsible for the presence of oxygen in the atmosphere, this is not stoichiometrically sound. There is simply not enough reduced carbon in the vegetation on the planet to balance out the vast amount of oxygen in the air. Put simply, the life cycle of plants is a part of the carbon cycle, but they are really dependent upon the atmosphere rather than the other way around. Burning off (oxidizing) all the plant life on earth would use up only a small fraction of all the oxygen in the atmosphere - less than 1%.
Chemistry must balance out its elements, that is what stoichiometry is. Trees cannot 'create' oxygen because matter is never created or destroyed, it merely changes form. When a tree absorbs CO2, it uses the carbon to build its structure and such, releasing the oxygen back to the air, but it does not 'create' it.
The vast ocean of oxygen in our atmosphere is a direct result of the fact that we have vast oceans of water. Water vapor is abundant in the atmosphere (where, incidentally, it is by far the most relevant green-house gas), and when it rises to the upper levels it is dissociated by high energy UV light into H2 and O2 gasses. The hydrogen gas is extremely light, and much of it boils into space, leaving the stable O2 behind to enrich the atmosphere.
Yet, it appears that there is no net loss of water from the earth due to the constant bombardment from micro-comets. These bodies are generally the size of houses, and the earth is peppered with tens of thousands of them daily. When they strike the atmosphere, they shatter into water vapor, adding water, and oxygen, to the ecosystem.
All life on earth is totally dependent upon the atmosphere, and it is dependent upon the supply of water on earth.
Posted 1322 day ago