Oxygen its what we breath, our most basic sustenance, yeast feels the same way. The best brewers oxygenate their cooled worts as a matter of course.
However in can be and is not good for our beer in other processes.
As with everything your grain should as fresh as possible but by by fresh thats up to at least two years old and 4 years or longer if stored cool and dry. For this we can thank Acsorbic Acid Oxydase which not only keeps our grain fresh along with other compounds but slows the oxidation of polyphenols and thiols in the mash.
However as soon as your grain is cracked oxygen starts it’s nasty work and gets to the AAO. A window of 15 minutes from crack to mash is considered safe.
The water that we pour is about 8ppm and at mash temperatures about 4ppm which is a significant amount of oxygen. We need to deaerate our mash water, the easiest way is boil and quickly cool to strike temperature (water absorbs oxygen from the atmoshere at 1-2 ppm per hour).
The grain should be added dry and the strike water under-let for least possible oxygen uptake, but it will happen during the course of the mash so to the mash sparge water add 30ppm potassium metabisulphite, this should be about right as we want to knock out any oxygen that finds, usually through agitation, its way in. It is also helpfull to add Brew Tan B at 1.5g per 20 litres of mash sparge water. The gallotannins in BrewTan B react with wort proteins and is highly effective at coagulating proline and thiol containing proteins which are involved in lipid and protein oxidation.
If you are recirculating then keep the outlet well under the top of the mash and also be very scared of how you sparge, and if you do use the drain out of the pipe method keep the bottom of the pipe just under the drained wort level by using a pulley attachment.