You’ll want a modified thinset if your tiles are porcelain.
I can’t imagine them to be ceramic
if they’re 36” x 6” and a specified floor file.
(I could be wrong of course)
Ultimately it’s going to rely on the amount of
deflection in your floor.
Here, we would not choose to tile on to OSB, exterior grade or not, we would want it replaced with a more suitable material or it would be overboarded with an inert flooring board,
(glued and screwed) Hardiebacker or cement board equivalent, or XPS type board like wedi.
Timber, no matter what type, is a poor choice of substrate on which to tile.
It’s subject to far too many variables as to whether or not your install will have any kind of longevity. (Expansion, contraction and moisture)
Timber is not a stable material.
A matting like Ditra 25 or your laticrete mat will only assist with lateral movement of your substrate, not deflection. (Side to side, not up and down)
Hence our course of action would be to exchange the OSB for something more suitable like 55/64” hardifloor not herdiebacker.
Failing that, the overboard route would be taken.
A quick deflection test would by to place a brim full glass of water in the middle of your floor and see if you can make the water spill by stamping on your floor in close proximity to the glass.
Better still a horizontal lined laser, and watch for the line to vibrate when the floor is stamped on.
If it vibrates, then there is too much deflection present.
The only other way to make your floor more stable without the extra height, is to add more cross braces between your joist, this should help reduce the deflection.
Ultimately, the choice is yours, especially as you’re installing your own tile, a professional should refuse in its current condition.
If you can remove all deflection and mat over the OSB with Ditra or similar using something like Ardex AF200 adhesive, then you may have a floor that will work.
But with a 6” x 36” tile, there is a high possibility it will have problems if your substrate is not the best it can be.
Now the choice, really is yours.
Good luck.
