Sealing OSB subfloor from moisture? Patching small gouges?
If you really want to stop moisture from getting to the subfloor then a membrane under your tile would be better than sealing the OSB. Better to stop it before it gets there than trying to seal the wood.

Tile and grout are not waterproof and if exposed to water regularly the water will work its way into the sub-flooring. 

Good luck!