Well, after finding a few oopsies on my side, it turns out that Oshpark likes one and only one NC Drill setup: God fearin', red-blooded, yankee doodle units in 2:4 format. Any other settings will blow up. Slotted vias will blow up all other holes on the PCB, so don't even go there, even though the capability exists in CircuitStudio now.
[None of this is a complaint against Oshpark. It's just not possible to do what they do and be all things to all people. I just needed to learn the secret handshake.]