My garage is a pole building type with pressed wood walls and exterior siding. I want to put sheathing on the walls for some insulation. The girts that the exterior wall is attached to are 1.5" x 4" being attached flat and run the length of the garage and are attached to the poles. Should I install the sheathing between the studs letting the studs exposed? Or should I attach the sheathing to the studs letting the air space between the sheathing and the exterior wall?
I can't quite picture what you're describing, but covering studs is always better than placing insulation in between when doing so is practical. Studs do not provide good insulation and turn into a heat sink. The dead air space left also provides a bit more insulation. Even better would be to insulate that space, then put more insulating sheathing over that and the studs.
If you don't need access to the girts (horizontally attached boards) then placing the sheathing over those, in between the poles, will provide more insulating value than placing insulation between the girts.
The more insulation, the better though, so doing both will be even better.
If you aren't stuck with sheathing, then you might also consider a spray on insulation which can easily and quickly go between and over the girts, and simultaneously provides a good vapor barrier.