[Europan Ocean (OP)]

I was watching a documentary on the emergence of insects from the sea, and times of the dinosaurs, when the atmosphere held a volume of 30% Oxygen. Why has it reduced to 20%, where did it all go? 

[evan_au]

The cause of the variation of the amount of oxygen in the atmosphere is not known

They do suggest that large volcanic eruptions can release sulphur, which reacts with oxygen in the atmosphere to reduce free oxygen...

I heard a suggestion that plants on land had free reign, until animals came and started munching on them. That might explain the first peak...
https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Third_atmosphere

[SeanB]

Got used up, converted to carbonate rocks in the ground and oceans, converted into biomass and water.