Hello, I am in the process of re-insulating my house.  I have removed the old siding and sheathing and filterglass insulation.  I am replacing the insulation with 3 inches of closed cell spray foam, sprayed onto the back of the existing drywall from the outside. The walls are 2x4.  I am also adding foil faced rigid styrofoam to the exterior side of the wall to prevent thermal bridging through the studs.  I am in Ontario Canada so we get extreme cold in the winter and extreme hot in the summer.  Do I need to add OSB or plywood to the framed wall before I install the rigid styrofoam boards.  Also if I tape all the seams of the rigid styrofoam, would I still need to add a Tyvek type air barrier.    

They say you can use steel straps to avoid the racking that might occur if you don't use OSB or plywood. With spray foam and taped rigid foam, I don't see much advantage to another air barrier layer.

My 30 year old house in NY has foam board sheathing. It is still straight. I do plan on redoing when I reside as it was never taped and I do get drafts. My plan is to leave the fiberglass and sheath with wood. Then add some closed cell boards on top and then firring strips to mount the siding on.

For your project, I'd put plastic sheeting on the drywall held with just a piece of tape at the top to keep it in place before spraying. This will allow you to remove the drywall in the future. It is not a vapor barrier which you won't need. Not having wood sheathing does cause problem mounting lights, shutters and other things so I vote you have some but it won't be needed structurally with the right foam sheathing. You will also have to decide if you want deep set windows and doors or deep indoors frames. I am going deep inside as it will look more natural and I don't have a rain issue.
Alaskans have some good sites where they have an acronym to describe the foamboard, cannot remember what it is but it show how to insulate including below grade.