OSB is a construction panel made with layers of precision manufactured wood strands that are aligned, formed into panels and pressed with an exterior grade adhesive resin. The aim of the paper is to provide a theoretical analysis of the hygromechanical behaviour of this material by using a two-step method. First, the mechanical and morphological parameters characterizing the wood particles, as well as the information concerning their orientations, are used in a multi-scale approach for the prediction of the behaviour of a single layer. Then, the classical theory for the analysis of laminates is used to obtain the stiffness constants of the panel. The flexural Young moduli of the panel as well as its in-plane hygroelastic constants are studied in relation with the orientation distribution followed by the particles. The study permits to establish that the orientation distribution function is a key factor in obtaining high structural properties for OSB. It is shown that an improved strand alignment may help to gain significant performance with respect to stiffness and behaviour under moisture conditions.