Fir plywood walls

I've used exterior plywood in an indoor finish application, and it worked out okay. I was able to sand  out the mill stamps with a random orbital sander. You might want to try and see if you can do that.

The exterior plywood we get here is oftentimes perfect--picking through the stack one can find clean rotary cut veneers with no nots or imperfections, no core voids, etc. Properly sanded, they can even be used for a certain sort of cabinetry. I made a kitchen out of them once.

No wonder that designers have been using them for interior finish. The same thing could be said for OSB. If you handle this stuff like gold, and detail it properly, it can look good.