Installing ceramic tile on top of wooden bathroom floor
I agree with OSB and leveling compound. I know it would be more work but I would see if the OSB is a underlayment and possibly only 3/8" thick, chip up a small hole where it would need to be patched if you use this surface. If a thin underlayment pull it up and replace with cement board. Cement board will not swell and fall apart if moisture gets under the tile like OSB will. Another advantage to cement board is it will be a better base for the tile making the tile stronger than on wood. I have dropped small tools and broken tiles in the past, but have found impacts on tile set on cement board with thinset resists breaking much better.
If you want to stay with the current base verify the "leveling compound is secure not loose and breaking up. Clean out all the loose stuff and clean up as much of the dust as possible. Once clean use some leveling compound mortar to repair the surface so it is smooth. Once the floor repair is dry it would be time to tile.