have you ever noticed whether or not cold crashing in the fermentor oxidizes your beer?
By the time I'm cold crashing, whatever vessel I do it in is sealed. There's no chance for O2 to enter.
air does vacuum in through the airlock. i do have a couple of those stoppers like steve linked above. maybe I should just use those...
Yeah, but why are you using an airlock when you cold crash? Fermentation is done, there's no need for it. I either keg to cold crash or seal the fermenter.