OSB has no place in a bathroom. Period. Use cement board on the walls. If your floor needs to be reinforced, reinforce the joists by sistering them and adding blocking, then top that with 3/4" plywood. Check a deflectometer to be sure that will be rigid enough. Then build your preslope. Then your membrane, then your tile.

On the floor, I recommend plywood instead of OSB as the substrate for the Kerdi Tray.

On the walls, 1/2" Kerdi Board can go right over the studs. But if you prefer to stiffen your walls with a layer of sheet goods prior to installig the Kerdi Board, I recommend plywood there instead of OSB for a couple of reasons.

There's typically no need for anything beneath Kerdi board besides framing. (On the other hand, I once demolished an addition where the homeowner had nailed each ceiling tile with 3" spikes for "added strength.")

Here is a link that might be useful: Kerdi Board applications

This post was edited by worthy on Fri, Sep 6, 13 at 17:35

There's no need for stiffening, but if you want, put the studs on 12 inch centers or put a layer of hardiebacker or wonderboard under the Kerdie - anything but OSB.

Worthy: "I once demolished an addition where the homeowner had nailed each ceiling tile with 3" spikes for "added strength.""

I had to take out a linoleum floor with 1/4" plywood underlayment that was gun-nailed with hundreds and hundreds of ring-shanks in each sheet - the installers had even shot-in stars and circles and spelled out their names with nails.

"We are amused!"