I want to know what the atmospheric composition was like 200 years ago
compared with today.  In particular what was the percentage of oxygen
in the atmosphere 200 years ago?  today its roughly 20%.

Oxygen (21%) and nitrogen (78%) concentrations have not changed
significantly.  But the carbon dioxide concentration in 1800 was about
280 parts per million (.028%), roughly the same as it had been for
thousands of years.  Around that time, because of the burning of
fossil fuels that came with the industrial revolution, the carbon
dioxide concentration began to increase significantly, and today the
concentration is about 370 ppm (.037%).  There are many man-made trace
gasses that do not occur naturally (e.g. tritium, helium 3, CFCs), and
they were obviously not present in 1800.  But their concentrations are
very low.  CFCs have brought about another important change however,
and that is depletion of the ozone in the upper atmosphere.  CFCs
'destroy' ozone, and have caused the so-called ozone hole over
Antarctica.  Meanwhile, the concentration of ozone in the lower
atmosphere has actually increased, because ozone is produced by
various human activities.