When wood becomes wet and then dries, restraint of shrinkage at the surface of wood by wetter surface and also sub-surface layers causes tensile stresses to develop resulting in micro-checking. Wood elements in composites such as oriented strand board (OSB) also swell and shrink when they become wet and dry. Furthermore, some wood elements are large enough to develop unbalanced surface and sub-surface tensile stresses. Hence, moisture changes and swelling and shrinkage of OSB might result in micro-checking. I test this hypothesis in this thesis. I also examine whether micro-checking contributes to thickness swelling of OSB. I used macro-photography, X-ray micro-computed tomography, and field emission scanning electron microscopy to probe the microstructure of OSB exposed to wetting and drying. These techniques were used to visualize and quantify the thickness swelling of OSB and the dimensions of micro-checks and other voids in OSB during and after wetting and drying. The spatial micro-distribution of a zinc borate biocide in OSB was also examined before and after samples were exposed to wetting and drying. Numerous surface and internal micro-checks developed in OSB exposed to wetting and drying as I hypothesized. Micro-checks developed during wetting, unlike the pattern of checking found in solid wood. Enlargement of voids was also observed during wetting and drying. Micro-checks occurred at the interface between latewood and earlywood and in the rays of softwood flakes and, less commonly, in the rays of aspen flakes. My results indicate that the pattern of micro-checking of OSB is different in some respects to that of solid wood, and suggest that micro-checking contributes to the irreversible thickness swelling of OSB. I briefly discuss the implications of my findings for the development of treatments designed to reduce the irreversible thickness swelling of OSB.