Oxidation in partially racked fermentor?

Racked half a 5 gallon batch from fermentor into a 2.5 gallon keg. Planned on bottling the remainder, but couldn't because my grandson woke up early from his nap. How soon will oxidation damage be a danger? Will the CO2 stay on top of the beer and oxygen on top of CO2?

No, they don't stay like that forever. They're already mixed at this point, my arbitrary guess is it would happen on the order of a couple hours if everything was undisturbed. I'm not sure if this will help at this stage (though you can try it) but if this happens again you can try adding some additional fermentables (briefly boiled DME or sugar) to the beer. The new fermentation will kick off enough CO2 to drive away the oxygen.

I'll have to try that if this ever happens again. It had crossed my mind to get the yeast going again to make some more CO2.

Thanks for the tip.