In Greece, large quantities of by-products are produced annually from urban and industrial activities among them very high olive oil productionby-products. Thus, the interest of the scientific community has focused on the utilization of olive oil waste mainly as a feed using various microorganisms and fungi, of which only a small number can biodegrade the waste. In the present study the effect of solid fermentation of olive mill waste (OMW)-straw (in various proportions) mixture using the LGAM P123 strain of Pleurotus ostreatus under the conditions of experiments on the quality of the olive pomace is investigated. LGAM P123 strain was grown in solid and liquid substrates as well as in a mixture of the hay and olive-pomace substrate. The results obtained from the experimental procedure resulted in an increase in the proportion of proteins in straw and OMW mixtures. In addition, the content of fibrous matter in the fermented samples showed a significant decrease whereas total polyphenols appeared an extremely strong decrease.