I know that it's just the garage but I'd like to repair this approximately 5 inch hole in an interior OSB partition wall. How should I proceed?
Turn off the power feeding that outlet and move the wires out of the way. Set the blade on your circular saw to make a cut just a little bit deeper than the OSB is thick. From the opposite side of the wall than what is shown in the picture, cut a rectangle out of the OSB that includes the top and bottom of the hole and extends to the middle of each stud to either side. Cut a piece of OSB to fit the hole, and screw it into the studs.
Alternatively, if you don't want to see a patch, you can just take off the whole piece of damaged OSB and replace it with a new one.
I think you'd prefer to do patch from inside only, yes?
You should have a solid, flush, matching patch when through.
PS A keyhole saw will enlarge the hole for the fewest $/use. I doubt any circular saw would maneuver in there. My personal favorite is one of the OMTs (Oscillatory Multi Tools).