Re: Garage to living space, osb question.
Just about any surface can be floated smooth with topping compound, sanded, then either painted or textured before paint, just depends on your tastes. I did a garage conversion last summer that was all plaster except for the new walls that went in which were drywall. All the plaster was floated and sanded smooth. The drywall was taped and finished to match, then everything was painted. It looks great!
The only cautionary thing I'll warn you about is fire rating. If you've got permits for the work being done, then you'll likely need to install 5/8" drywall over everything. If you don't have permits, then you can do as you please, but be aware that OSB will have a much shorter burn through rating than 5/8" drywall. Also, a lack of permits does not mean that you can be lax on following building codes. Should there be a problem, your insurance may not like non-permitted work.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!