After a decade of using OAV, this is my practice::

Best outdoor air temp: about 45-48F. Yes, the bees are clustered, but a critical prep step is to 
a) get down and look into the entrance of  hive to see where the cluster is, and then,
b) use enough smoke to loosen and shift the cluster upward and away from close contact with the business end of your device. I like to hear 'em grumbling.

I have occasionally done a treatment below 40F, but it's uncomfortable for the operator, prematurely exhausts my battery's power and any bees that fly out will quickly be chilled.

Don't worry about remaining patches of brood if you're going to do it now.  But I don't think it's a one-and-done sort of thing, which is why I am unconcerned about patches of brood.

I do a fall series of four treatments on five day intervals in mid-late October (it takes 16 calendar days to do a round of 4 treatments on 5-day intervals). This knocks back the remaining in-hive mites and also is a remedy for newly-arrived mites due to robbing. 

But, in my opinion, this is just the fall treatment phase of late-season mite control with OAV .  You're overlooking a critical element of next year's mite control program if you don't repeat this series at the end of the year. Because in the north, once the brooding has truly stopped AND after the cold temps will restrict your bees to their own apiary (even if they go out occasionally in the winter) your OAV-cleaned-up bees will stay nearly free of mites for 2-4 months,  or until new mites can find their way in after foraging recommences in late March or early April. This extreme reduction of mites allows your first rounds of spring brood to be born into as low-virus environment as possible. And it starts the season from as near-zero as you can achieve, dramatically lowering the starting point for the inevitable upward curve in the mite population.

I think of the second treatment series (I aim to to start around the Winter Solstice-  Dec. 21st.- here north of Albany, NY) as my annual Christmas present to my bees.  For me, it's not the last treatment of the year, but the one that sets the conditions for entire next year's mite control battle.

Nancy

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