A two-part flame-retardant, a flame-retardant oriented strand (OSB) and method for forming a flame-retardant OSB is provided. The two-part flame-retardant composition includes an aqueous solution containing a water-soluble flame-retardant and a flame-retardant powder that is incorporated into an oriented strand board without substantially affecting the mechanical properties of the oriented strand board. The method includes applying the aqueous solution containing a water-soluble flame-retardant to an oriented strand board furnish and applying a flame-retardant powder to the wetted furnish, without requiring an additional drying step.