[onewhippedpuppy]

Remember that OSB is frequently used for exterior sheathing on homes. So while it isn't actually directly exposed to weather, it is frequently used in a non-climate controlled environment. I've even seen people use it on cheapo sheds where it gets exposed to the environment, it holds up surprisingly well. Overall it's really tough stuff.

The corrugated steel roofing/siding is also a great garage idea, my father in law used it on his shop ceiling and it turned out great. I might be a little concerned about denting if used for a wall, and the corrugated finish and thin metal wouldn't be great to hang stuff on.

If you buy masonite sheets to paint, you would have to paint the unfinished side or rough up the finished smooth side. The finished side is a semi-glossy surface that won't take paint. Though you could buy 4x8 sheets of masonite siding and paint it.