The O2 comes from algae and cyanobacteria photosynthesis. Algae and bacteria
are more effective because of the chemistry of photosynthesis: the carbon is
used from the air to create carbohydrates, which are either reused
(releasing CO2 again) or are used for building the plant structure. Woody
perennials, like trees, need less for the plant structure while short-lived
algae and bacteria need the most (high school biology).

I don't have any links for the relative amounts - I haven't looked that up
in years - but I'm quite sure it's accurate. Well, not the "evaporation"
part, but I have trouble with words at times, too! Currently I'm looking for
world livestock census for the last century or so, looking for the source of
atmospheric CO2 rise. I've pretty much ruled out deforestation as a major
cause. That's partly for the reason TeGGeR cites, partly because
deforestation of the Middle East (which was on a far larger scale than South
American deforestation has been) didn't produce the rise we are seeing, and
partly for lag times and other numbers issues. If anybody has any good
sources on that, I'll appreciate them.

Mike

Free oxygen and oxygen bound with hydrogen atoms are not the same thing.

Water (bound oxygen/hydrogen combination) also contains free oxygen that is
not bound. It is this which is released on evaporation.
http://www.agu.org/revgeophys/sleep00/node8.html

If you were to try to crack the water molecule into its constituent atoms,
this would requires vast amounts of energy. More, in fact, than the
hydrogen itself would provide.

In spite of certain school texts stating that the rain forest provides up
to 40% of the earth's oxygen, plant-generated oxygen is not a significant
source of atmospheric oxygen. Source: Facts not Fear, by Michael Sanera &
Jane Shaw. Regnery Books, 1996.