... Various studies that investigated the influence of oregano (dried leaf and oil) on the performance of broilers have led to the conclusion that oregano contributes to improving the performance of broilers by promoting their growth (Giannenas et al., 2003;Halle, 2001;Modeva et al., 2003;LiHua et al., 2007) and can reduce bacterial gastrointestinal tract populations such as Clostridium perfringens and Escherichia coli (Giannenas et al., 2004;Fukayama et al., 2007). However, the prediction of the oregano broiler's response is not very simple, because it is influenced by the variety of the plant (Fritz et al., 1992;Lee et al., 2003;Demir et al., 2003;Hassan et al., 2004;Halle et al., 2004), the level of inclusion in diets (Giannenas et al., 2004;Alçiçek, et al., 2003;Alçiçek et al., 2004;Ertas et al., 2005), sanitary and environmental conditions, nutritional composition of diet (Fritz et al., 1992;Fritz et al., 1991;Jamroz et al., 2006) and possible interaction with other additives. The literature data show distinct biological functions of the essential oils derived from aromatic plants, such as antibacterial, antimicrobial properties (Chen et al., 2016), antifungal (Hossain et al., 2016), antiviral (Gavanji et al., 2015), antioxidant (Shaaban et al., 2012) and antiproliferative properties (Park et al., 2014). ...