Changes in the air permeability and density profiles of 12-mm-thick oriented strand board (OSB) specimens were evaluated in relation to changes in their moisture content. The test methodology consisted of the simulation of real conditions that may occur during construction. Using a water bath, the OSB moisture content was increased from 10% to 17%, and the consequent changes in the air permeability and vertical density profile (VDP) were analyzed. The air permeability and VDP were then reanalyzed after acclimatization of the OSB to a balanced moisture content at 60% relative air humidity and 11.4 °C. After wetting the boards with an initial moisture content of 10% for 2 h and naturally re-drying them in laboratory conditions, an average increase of 11.7% in air permeability was observed. The increase in air permeability was 5.6% with a pressure difference of 50 Pa. After redrying, the boards showed a 1.1% lower average density and 14.5% lower maximum density in the surface layers. From the results, it followed that even the short-term effects of water and the related increase in moisture content of the OSB had a negative impact on the air permeability and VDP.