Now the issue we've run into is that if we lay Ditra, it will raise the bathroom floor above that of the hallway (which we'll do and use a transition strip if there are no other alternatives--adding a layer of plywood would raise it also). A tile guy is looking into whether there is a "primer" (his words) that can be applied over the OSB to not increase the floor height. I'm wondering if that is perhaps Redguard. What about using Kerdi like davidro1 referred to above in his post? If that's usable, how much does it raise the floor? So, are there alternatives to Ditra that won't the raise the floor height as much? Many thanks again...

In my experience, the main reason MDF/chipboard/OSB subflooring squeaks is due to internal separation of the product. I have worked on several homes with subflooring listed above and was not able to fix all the squeaks. I used several products designed to insulate/separate the subflooring material from joists, screws designed to be installed through carpet, through the subfloor and into floor joists, and adding fasteners. The only sure fix was to remove the material that squeaked and replace it with plywood.

I do not know the brand or thickness of the OSB. My project is a remodel of a builder grade bathroom/home built in 2002. Nothing has been demoed just yet, although the OSB flooring, which still has original flooring (1/4” unknown type plywood with linoleum atop) in-place, will not be removed (as long as in good condition). The deflection rate is also unknown. I do not reference cement board from a rigidity standpoint as it seems to have been misunderstood; it WILL also be part of the mix per my preference. Last, the tiles will be porcelain tiles. Respectfully, my post generally focuses on: 1. What is the purpose of the thin-set mortar between an OSB subfloor and 1/4" cement board? 2. What type of thin-set mortar should be used (non-modified vs unmodified) between an OSB subfloor and 1/4" cement board? These inquiries have been made given manufacturers' data sheets explicitly state not to apply polymer-modified thin-set or unmodified thin-set to OSB. As a result,I would appreciate insight into my explicit inquiries, if able to provide.

Need to determine if the floor system currently there can handle the increased weight of the tile (deflection) before you even attempt to determine if it can be applied over OSB. It can be done with the proper Ditra or similar membrane installed over the OSB