Exterior paint has fungicide in it to prevent mildew from rain/snow/fog. If the interior of your garage is so wet you need fungicide you've got more problems than just paint selection!
There's nothing wrong with using exterior paint inside a garage, it's simply not needed.
An oil primer with latex topcoat is a great combination. However, modern latex primers are as good as oil primers in many applications. A water based primer will swell OSB though, so you should only use an oil based one here.
Want to save money? Use a PVA (polyvinyl acetate) drywall "primer/sealer" for the drywall only. It has minimal hide but seals the paper facing and gives "tooth" for the topcoat. A PVA drywall sealer is cheaper than regular primer because it has less pigment in it. New drywall doesn't need a high hide. Old drywall with discolouration needs a stain blocking primer, or you'll be using multiple top coats to get an even colour.
So, use an oil based primer on the OSB and lay it on thick: it will absorb a lot. Use a drywall primer/sealer for the, um, drywall.
Then top coat everything with the best quality latex topcoat you can afford. 100% acrylic is best, but don't pay extra for zero or low VOC paints, as their only benefit is less odor which is useful for hospitals, schools, etc but not necessary in a garage. Top coat with two full coats. The second top coat will use less paint than the first. If you do these steps you won't have to paint again for a very long time.
