Flooring Tile - setting 1/4 hardiboard over 3/4 OSB subfloor??

I am about to set a slate tile floor in the bathrooms of my house. Subfloor is 3/4 T and G OSB on joists 16" on center.

1. Can I put down the thinset to set 1/4" backerboard directly on top of the OSB or do I need to add treated plywood to subfloor???

2. What potentially could happen to the floor if I do decide to set the backerboard directly over the OSB??

Dont use treated plywood, only that which has an exterior glue rating such as BCX or ACX is to be used. It would be better to use the 1/2" ply plus the 1/4" backerboard to reduce the "between the joist" deflection that would be present otherwise. Cement boards offer no structural strength to a subfloor, only an additional layer of exterior glue grade plywood of 3/8" or thicker will do that. You are at the bare minimum for subfloor stability using ceramic tile. With the stone, such as granite, marble and slate, there is a higher threshold to meet to avoid tile cracking in the future, hence the recommendation for the additional ply. FYI, cement board can be set down onto OSB in a bed of thinset and then anchored with galvanized roofing nails or backerboard screws. Not so with pressboard, which will disintegrate when exposed to moisture.

Thanks Bob!!

One more question-- whats the best way then to make the transition from 3/4 hardwood to the slate tile sitting on the ply plus the 1/4" cement backer??

The wood flooring companies make some transition pieces, one looks like a clamshell molding. Another is about 5" wide and bridges the gap between the wood and tile. Plus there are several other types also.