Unscientific OSB/Advantech Bucket Test Results.
I submerged one piece of 7/16" OSB 6"X6" and a piece 5/8" Advantech 6"X6" in a bucket of extremely cold water for about two weeks and the results are in. The exposed edges on both samples swelled about 1/8". I then sawed the samples in half. The OSB was saturated the entire width of the sample and the center of the sample had also swelled about 1/8". The Advantech 1/8" swelling extended inward from the cut edges about 1/2" and then the Advantec swelling was minimal at about 1" from the edges. There was only a slight bit of swelling on the wood fibers on the surface in the center of the sample. From about an 1" from the exposed edges the swelling of the Advantec was only an appearance issue since the core of the material had not been penetrated.
Bottom line: Any wood fibers directly exposed to water will absorb water swell and then rot.
OSB: Longer wood fibers, less sealing resin and voids in sealing making the material almost completely porous. Plus side, it weighs less.
Advantech: Shorter wood fibers less capillary absorption, much more sealing resin making it almost non porous on sealed sides except for the very top of the surface. This material is much stronger with less flex than OSB but it is heavier. The surface of a new sheet of Advantech has a waxy feel I would definitely sand any places you wanted to
fiberglass in and I would also seal any cut edges.
The above was my observations and not an endorsement for either material.
Eddie