Sealing an OSB floor in a shed
Why not consider putting down a sheet of vinyl similar to what might be used on a kitchen or bathroom floor. 

You would have to ensure that the underlying OSB flooring material was fairly solid and all joints supported. If it is just one layer of OSB you may want to start with a second layer of OSB that has the sheets installed in a manner that no joints in the first and second layer are in the same places.