I used to crash in the fermenter with an airlock but have switched to crashing in the keg under pressure. Although I noticed the suck back into the fermenter, I never noticed any oxidation on my beers. My theory was always that the CO2 in the fermenter provided a blanket over the beer, keeping the air from being an issue. Regardless, by crashing in keg under pressure I have no worries. 30-32F for 2 or 3 days.