You have more confidence in OSB (or as I call it "Chip Board", among other
names).

OSB begins to become unstable even near the presents of moisture. Even if
you cover it inside and out, the moisture will find it's way in and crack or
break the seal of anything you coat it with..

Additionally, OSB has very little tensile strength, an important
characteristic in any water craft. To over come this you would need very
closely spaced frames and stringers, thus adding weight and cost.

My comments are not intended to "rain on your parade". If you must
economize, try using CDX play and your plans would have a much better chance
of success.

My opinion and experience, FWIW.

Steve
s/v Good Intentions