From Randy Oliver:

"OA does not exist as vapor when it touches hive surfaces or bees, otherwise
it would melt or cook them.  Instead, it forms tiny crystals.  There is
very clear evidence that if a mite's footpads touch those crystals, the
mite dies.  If a crystal touches a mite's carapace, there is no apparent
immediate effect."

I thought I made my understanding of crystals forming on the surfaces of the hive clear with my next question about putting bees into an already treated nuc box. Assuming that your carapace statement is correct, (and I'd love to read the study), then there should not be much difference if the bees/mites are present for the crystals being deposited, so putting bees into a pre-treated nuc box should work just as effectively. ***Unless the composition of the crystals changes (absorbs water?) or degrades over time***

I agree as well with Bill's statement regarding the type of the oxalic acid used. My memory of organic chemistry is a number of decades old, but I am well aware of the issue of bio-availability when it comes to treatment effectiveness. 

All of this information has practical implications in terms of maximizing the effectiveness and efficiency of treatment options. And given Randy's great article about nuc build-up in the recent ABJ, and the very useful mite model he has put out, I feel this question has merit. 

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