Is there such a thing as too much oxygen in the reservoir?

Yes & No. Yes, because if your water pump sucks air it will fry the water pump so be careful when placing air stone so they are not to close to the water pump. No, because the root system must have oxygen to live, as far as the root system goes the more oxygen in the water the better. The smaller or finer the air bubble the better, because the smaller bubble has less surface tension making it easier for the roots to absorb. Having good air circulation on the reservoir also keeps your nutrients mixed instead of settling down to the bottle of the reservoir.

Air bubbling in reservoir.