If you do not apply internal pressure to the keg while cold crashing I believe it will pull O2 into the keg.
Kegs aren’t designed to hold a vacuum, they’re designed to hold internal pressure, so as the liquid cools air will leak into the keg around the seals just like suck back thru an airlock.
Of course, just like everything I post, this is simply my opinion. Others may disagree which is certainly their prerogative.
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