Can you replace the OSB with 3/4 plywood? I like OSB for a lot of things, but it isn't very rigid. I'm sure you'll be fine with it in the end, but if you can do anything to make it stronger, now would be the time. What about additional framing behind it?
Eddie
Layering plywood would be the easiest but it also wouldn't be too hard to replace the osb. I'm out of town tomorrow for work but will take a look at it on Sunday.
Jeremy