On earth, lightning removes oxygen from the air by combining it with nitrogen.
Not to mention all the oxygen it removes by starting fires. I further believe that lightning seldom if ever penetrates into bodies of water or otherwise does electrolysis. Falling water is not grounded and grounded water is outside the current flow, which is from the highly charged surface and the oppositely charged cloud region.
We can be sure that there was little oxygen in the atmosphere on earth until life got started, notwithstanding all the lightning. All the banded iron formations date from before the oxygen levels of the atmosphere rose. They couldn't form and haven't formed since.
Very good. But then what should be the constituents of a lifeless atmosphere, where liquid water is present along with lightning and of course a sun?