I just came back from my building supply store and they didn't know what I was talking about when I mentioned CDX plywood. I'm looking at putting 3/8 ply over my 5/8 osb subfloor. I told them that the C meant C grade surface and they told me that it is probably sheathing... is this correct. 2nd they sell 7/16 OSB for almost half the cost. Can one use OSB as an underlayment.

Thanks for all your guidance.

It's best to use plywood imo. CDX is also known as sheathing plywood.... but they should know that for heavens sake Flip a sheet over and it will be stamped CD or CDX.

C - face layer
D - bottom layer
X - exterior glue was used

It also comes in 3ply or 4ply. I quit using CDX myself because it's getting sorrier. The inner 1 or 2 plies are full of voids you can't see. I prefer using BCX because it is an underlayment grade made for this purpose.