Oriented strand board (OSB) is a well-established wood based panel product. The production and consumption has strongly increased during the past decade. It is well known that most of the wood based panel properties are strongly related to the specific gravity of the panel. The EN 300 requires that the specific gravity within a panel or the horizontal density distribution (HDD) is within ±10%. For identification of different influences of raw material and process parameters, computer simulation programs of HDD were developed under laboratory conditions (Dai, Steiner 1994a, b, c). In this study commercial strands and OSB panels are investigated to verify and improve the suitability of the computer simulation models for industrial conditions. First, a wide range of strand dimensions is determined where strand width seems to exhibit the greatest coefficient of variation. Second, an analysis of the density within an OSB panel of 8 ft by 24 ft indicates that HDD clearly exceeds the EN 300 limit. Finally, the computer simulation program was improved by considering the effects of strand types (dimensions and shape), amount of fines, strand orientation, and compartment of strands due to orienting discs and rollers during mat forming. Results of the improved computer simulation correspond accurately to the experimental HDD. The model predicts that the fines and the strand compartment strongly affect the panel density distribution.