[kupfer]

From their FAQs:

>Which material is better, ply or OSB?

>Ply is lighter and generally better structurally but more expensive. OSB is cheaper but heavier. With the recent research on WikiHouse Skylark we did test both materials, but in terms of the full spanning floor beams it’s clear ply offers advantages in terms of strength but also because it’s lighter it’s easier to move and carry. A hybrid approach is also a possibility.

[LegitShady]

He's talking about the moisture resistance property of plywood over OSB. OSB absorbs moisture readily and turns into mush.