When they tear off the siding to add 1 or 2 inches of exterior
foam-board insulation, is it typical (or not, or it depends) to put a
layer of house-wrap under (or over?) the foam board?

Since we're talking northern climate (think Detroit / Buffalo / Toronto)
and since there is already a vapor barrier on the back side of the
interior drywall, I wouldn't think that its a good idea to have a second
vapor barrier anywhere, especially closer to the exterior.

The exterior sheathing is some sort of drywall / gyprock or sheetrock,
presumably exterior grade, and it's covered with a couple layers of some
sort of thin black paper (paper has a waxy or oily feel to it, tears
easily).

I'm thinking of taking the paper off, sealing the seams between the
exterior drywall panels, putting up some 2" expanded polystyrene 4x8
panels, and then covering with 7/16 OSB. I figure the OSB will be able
to sandwich the foam good and tight against the drywall, removing air
gaps, and giving a good attachment surface for the siding. This is for
the second story, the ground floor has brick exterior.

Comments?