Hi, Sarah. First, let me say that I am not a structural engineer but I have used a lot of wood over the years. And, honestly, I do tend to overbuild, but I also am usually pretty tight with a penny. I tend to be the only one in the house that knows that lights also turn off, for instance.
I still don't like 7/16" OSB for anything other than things like siding or subfloor, where it is totally supported or gravity can't really work on it. I have some basement shelves made of OSB that I did not build that sag beyond all belief. I think that you could probably get away with the 1/2" CDX but, if it was I, I would spend the extra $50 and go with the 5/8". Yes, you could put 2x4 bracing on the edges of the sheets, and I probably would with the 1/2" CDX. With $50, you could buy 15 2x4's and some nails and that would be more than enough to do the job. But I don't think that you have to put them in with the 5/8" so it really comes out a wash or darned close to it. And you don't have to cut and nail them in. Laziness wins. Just my $.02, but that's the way that I would go. And I would run the sheets the long way and stagger the joints so I had nothing less than a half sheet on one end. Wish I could draw a picture, but, hopefully, you get the idea.