Agricultural wastes from Gossypium hirsutum and Gmelina arborea sawdust were used to cultivate a mushroom, Pleurotus ostreatus. Three hundred gram (300 g) of dried substrates were weighed and mixed individually with 10%, 20% and 30% concentration of additives (rice bran and corn cobs) and loaded into polypropylene bags. Each treatment was replicated thrice. The bags were tied and pasteurized in a drum for 4 hours. After cooling, each bag was inoculated with 10 g of vigorously growing spawn of P.ostreatus. The bags were incubated at 28 ± 20C and observed for growth parameters and fructification. The results showed that G. arborea sawdust with 20% rice bran additive gave the highest stipe length (9.0 cm), pileus diameter (7.7 cm) and stipe width (8.9 cm.) The proximate composition of the harvested fruit bodies showed highest protein content of (24.97%) in G. arborea sawdust with 20% rice bran , highest fat and ash contents (7.72% and10.95%) in G. arborea sawdust with 10% corncobs and highest crude fibre content (8.49%) in G. arborea sawdust supplemented with 20% rice bran. The highest moisture content (86.53%) was found in G. arborea sawdust supplemented with 20% corncobs. The highest carbohydrate value of (17.11%) was recorded in G. arborea sawdust (control). The highest total yield (95.44g) and highest biological efficiency (31.81%) were recorded for G. arborea sawdust plus 20% rice bran. This study shows that Pleurotus ostreatus cultivated on G. arborea sawdust with 20% rice bran incorporated as an additive produced better and bigger mushrooms.