The earth's original atmosphere formed as the earth first formed from accumulating planetesimals. The heat of planetary formation "cooked off" much the hydrogen and helium, driving them into a gaseous state. However, the earth was too small and too warm to retain these gases, and this original atmosphere was gradually lost to space.

Ongoing volcanic activity produced the earth's secondary atmosphere through outgassing, with some additional contribution from impacting comets. The gases released by modern volcanoes average greater than 50% water vapor, about 25% carbon dioxide, and 13% sulfur, with smaller amounts of nitrogen and other gases. Evidence of life on earth dates back at least 3.5 billion years and possibly over 4 billion years, and an atmosphere dense enough to allow water to remain liquid must have been present before life developed. Given the molten state of the primordial earth, volcanic outgassing probably began as the earth was forming, so the atmosphere is likely to be as old as the earth itself (although biological contamination with oxygen has changed its composition).

from the earth being billions of years old... it took until 3-3.5 billion years ago for life to start on earth. Then as those early prokaryotes proliferated, it took another 1.5-2 billion years for the predominantly carbon dioxide atmosphere to convert to be more oxygen rich. This was a result of early bacteria developing photosynthesis which used CO2 and expelled O2 into the atmosphere. Then the life on earth had to adapt to the oxygen environment we have today....

Rusted Iron in fossilized earth did not appear until 1.7 billion years ago so it is safe to say that oxygen was not present when the earth was formed.... And oxidizing atmosphere was a result of the earlier atmospheric changes of earth.