Please don't confuse OSB with the sawdust type product used by the building industry for a smooth surface for vinyl floors or the base of counter tops.
Oriented Strand Board (OSB) board is quite secure, you'll see it being used by builders for roof construction, exterior wall sheeting, and sub-flooring.
Particleboard on the other hand soaks water like a sponge. It often is used for the base for counter tops when they are laminated with some form of Formica. counter tops.
BUT as bobby A said above... water will damage any type of wood product. Yes even exterior ply wood and pressure treated wood.
Here is a good link to information of the product... please check out the
Frequently Asked Questions
http://osbguide.tecotested.com/osb