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You probably only would like to do this once, so therfore get 5/8 min. if you are using 18 inches of insulation. If you buy the better quality OSB it will be uniform. There are different grades, and the color of the paint on the sides tell the story. The ones that are a different thicknes can be bought cheaper than the better grades. Usualy the big box stores sell the iregulars. Now look on the printed side. Usualy other than the 9 foot stuff there is a story to read. One of the most important words are 1/16 gap on sides and 1/16 gap on the ends. A simple nail will give you the gap---1/16 for expansion. You sometimes hear about OSB bulging, well again--there is a reason it is stamped on each sheet. We sometimes make batten strips out of the scraps. If you like painted then paint the ceiling first or just seal it then paint the batten strips a different color if you desire. --while on the saw horses before you rip them saves alot of time as well. Give them the second coat and paint the dedges the second coat, and then nail them on.
I am a big hard headed old timer that actualy think that OSB was once in its most usable state when they chipped it up, and before adding the rosin to make sheets out of it. Mom could spread it around the flowers and I could nail real plywood on the buildings. Those days are gone and we are forced to use OSB instead of plywood.
As others have suggested --most building codes do not call for OSB for a fire stop, rather it is sheet rock. Your insurance Company, now or in the future may make some demands on your OSB idea. Next question is have you ever been around a building that burned down? Yea, it can happen to you---price out 5/8" sheetrock and give it some serious thought. Regardless of your decision rent or borrow a sheet rock jack. Sheet rock jacks make good friends. If you are going to paint it make use of your pre painted 16" OC and 24" OC lines--rough side down. That saves alot of time. They will always be 16" on center and the center line will be for your rafter two foot spacing.
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