Smoothing OSB plates
Straightforward enough job.  We stabilize wood like that all the time.  

I'd start by painting the piece with a thin coat of either Amercote or West System Epoxy, just to freeze the splinters so they won't keep redeveloping as you prep.  Then a very thorough sanding with a jitterbug sander until there are no high spots whatsoever.  Then one more shot with the West System, then a sand-down just to knock the high spots off, if any.  Then prep &amp; paint. 

How much money did you say you'd be saving by using OSB instead of finish plywood?