Why did the builder install an OSB Wall behind Drywall?
That is probably what's called a shear wall. It's laterally structural to support the building's side-to-side forces.

Penetrations in your shear wall must adhere to different restrictions than other walls and you may need to consult an engineer to verify. However, I suspect that if you're just removing enough osb to accommodate a small sub panel you're probably fine, you may need to block around it.