Welcome to the forum,

  OSB is a poor choice. Simple exterior plywood is a minimum and all faces should be fiberglassed to prevent 'checking' (raising of the grain and splintering)and to protect it from the water.

For boats or even just long term exposure to the direct weather the choice is marine plywood. This is plywood for which all voids, knots or holes, are cut and replaced with wood inlays and a special water proof glue is used to hold all the plys together. Unless the voids are replaced with clear wood material there would be areas with a different strength or flexibility and an are where moisture could accumulate and eventually rot the surrounding wood. Also the plys of the plywood build up are rotated 90 degrees to the neighboring sheets to create a weave pattern like woven cloth to provide both strength and flexibility to the sheet goods. A special waterproof glue is used to bind the sheets together so the glue does not erode from between the plys.

OSB most likely will not use water proof glue, might not have the alternating weave pattern, and could have large voids or large concentrations of brittle glue. So the material might not be as flexible as marine plywood and might not weather as well as marine plywood.

You could test this by getting a piece 1/2 inch OSB, B/C plywood and marine plywood. Place the 3 pieces outside and observe what happens over time. I would expect OSB would fail first, followed by the B/C plywood, and then the marine plywood would fail. You could also test the flexibility of each piece and note the differences.

I've used OSB in construction and watched it weather in rain. It holds up fairly well but starts to flake after extended exposure. I would think one big drawback is its texture, which would create more drag than plywood when used in a boat. OSB is interesting stuff and we use it a lot in California to create more earthquake resistent structures, but in important shear walls, codes often require plywood. So I think OSB is not as strong as plywood.

I have several thoughts about the original post.  OSB (Oriented Strandboard) is a relatively new, engineered wood product intended to replace plywood in the housing industry.  When OSB is kept away from moisture, before, during and after installation, it is a viable choice to replace plywood.  So, if you plan to build a boat with flat walls, flat floors and flat roofs, and you can be sure the OSB won't get wet, go for it.  And yeah, some OSB can be better than others with respect to moisture.  It depends on how the manufacturer created the panels (the wood, wax and glue ratios, chemistry, heat and pressure, etc.).

Marine plywood hadn't been invented by the 1930s.  Plans for home-built boats of that era specified external plywood in the hope that the ply used would have a waterproof glue holding the layers together so the plys wouldn't delaminate, a common problem back then.  Not taking advantage of all the benefits modern marine plywood has to offer today (e.g., lighter weight and consistency), to build a racing sailboat, would be silly.

Epoxy and fiberglass weren't around in the '30s, either.  Use of these materials can increase the strength and durability of a small craft significantly over what was expected from a home-built boat in the '30s.

Marine ply, epoxy, and fiberglass will cost more, initially, than the materials listed in 1930s' building specs, but they offer a clear benefit, especially for a race boat, and will be more economical in the long run.  Good luck with your build.

The biggest investment is your time. Use marine plywood. Even better, and recommended... use BS-1088 plywood. If you are going to use the boat for more than one season, the small extra cost of using better materials will pay for itself a hundred times over. Also, construction will be a lot easier with plywood that lends itself to being bent in stitch and glue methods.

Keith

I would not reccomend it usuing OSB on any boat. I have a 10' Lowe aluminum  jon boat that had a transom made out of OSB (from the factory) and it turned to mulch after a few years of exposure to moisture.