When I read "new particleboard" I thought you must be misidentifying it, as no one in their right mind would use particleboard for a subfloor. No one with any mind at all would use it for subflooring in a wet environment.
Do get a positive ID, because if it is particleboard you should consider ripping it out and replacing it. A bit of work, but not as much work as it will be two years from now when you have to rip out your tile and your particleboard.
Particleboard looks like coarse sawdust held together with glue. OSB looks like wood confetti (1-4" pieces) held together with glue.
In a wet environment, again, I agree with Tom. Subfloor, then cement board screwed to the subfloor, treat/thinset the seams, then tile with thinset. Not mastic.