Olive trees cultivation is one of the important agricultural product in especially Mediterranean, Aegean and Marmara regions of Turkey. There are about 750 million productive olive trees worldwide whereas 98% of them located in the Mediterranean region. Three major olive oil producers worldwide are Spain, Italy, and Greece, followed by Turkey. Olive Oil Mill Wastewater (OMWW) is one of the waste products of olive oil process that contains different harmful substances such as polyphenols and long-chain fatty acids which risky for soil and plants. These harmful materials are environmentally not safe, while they cause economic and ecological problems. Due to the considerable amount of OMWW occur every year, a number of approaches tested by the researchers to overcome this issue. In this study the effect of OMWW alone and combination with green manure, rhizobium and humic acid on soil microbial activity evaluated. Results gathered revealed that OMWW was reduced determined microbial activity parameters as soil respiration (CO2 production), dehydrogenase enzyme activity and microbial biomass carbon content. All applications were effective on restoring CO2 production that diminished due to OMWW; however, humic acid and green manure + humic acid applications yielded the highest benefit. Dehydrogenase activity did not differ strongly by OMWW applications. Microbial biomass carbon (MBC) values decreased rapidly by OMWW incorporation, but green manure and bacteria application together restored MBC, even higher values determined in that variant.