T shaped filler for osb gap?

I would cut the OSB to fit the gaps on the joists. Any attempt at fillers is likely to be disappointing as the material sags over time at places it is not supported.

OSB is cheap, a little cutting to fit shouldn't be to painful.

Just cut the sheets to fit the joist spacing.

Depending on the waste it may be just as cheap to toss 2x4's up to match the 4x8 boundaries as needed. All we know is that joist spacing doesn't lay on 48 or 96 inches. Also we don't know shop size. Depending on your area it may be different, but where I am a 2x4x8 is roughtly 1/3 to 1/5 the cost of a sheet of OSB so it can be cost effective to add a little framing.

Look in the panelling section at a BORG for filler strips. Those are, however, likely to be cosmetic not structural.

hex

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I used OSB in my shop in place of drywall to provide a little bit more durability against damage from things bumping into it on occasion. To finish the OSB, first step is to insure you have the smooth face exposed, then I just used drywall compound and tape or webbing. In some cases there were a few face side blemishes showing so I just filled them with drywall compound as well, then after you are satisfied with the surfaces showing, I primered the wall with zinseal bullseye followed by a latex topcoat. Hope this is useful, regards, Joe.

It'd be easier to cut slots in the edges of the OSB (I'd use a table saw for this, if the OSB is handle-able on edge), fit a spline into the slots, and make tongue/groove joints. Once the first panel is installed, it's gonna be a lot easier to hold up the second if they interlock.

I dislike the sag potential of OSB on 4' centers, though; could you add supports to get it to 2' ? And by all means, prime and paint both sides of the panels to discourage moisture movement. The backside can take any bargain/offcolor/leftover paint you have, of course.