I know I am not alone in discovering the secondary ferment has gone off because its oxidised. You know the signs, vinegary smells, a milky film covering the the surface. Professionals have an amoury of tricks to avoid this such as sealing the barrel and inserting various gases. The amateur can mimic to a point with CO2 and ascorbic acid or crowd out head space with sterile objects to displace the liquid. I find that is the best tactic..reduce the head space either by displacement or making a bigger brew. And adding some extra sugars to encourage CO2 at racking time.
Also, and I know this doesn’t follow conventional wisdom, I deliberately aerate by rousing several times during the primary ferment. Then, when the krausen has settled a bit, instead of syphoning to avoid exposure I bail and pour the whole batch into the secondary fermenter splashing so as to provide oxygen to restart the ferment. I picked up this idea on an internet forum and tried it out and have been very happy with the results.
A final thought, this time about hot side oxygenation. If this does happen it occurs at a level too subtle for my taste buds. In fact if it does occur I would have thought it would be during sparging but I’ve not seen anyone suggest that is a problem. Mind you the subsequent boil should remove any oxygen