Pythium genus includes significant and destructive soil borne plant pathogens causing losses on many plants, especially by root rot and damping-off diseases. Like other soil borne pathogens, control of the diseases caused by this group is rather difficult. Usually fungicides are used against Pythium species, but investigations on alternative methods and chemicals gained importance because of the harmful effects of pesticides on human health and environment. Plant extracts and essential oils are among the safer alternative chemicals. Since oregano oil has high fungitoxic effect even in low concentrations, effect of oregano water, by-product of oregano oil, was investigated in this study against P. deliense, one of the most common and virulent species in Turkey. Oregano water obtained from Origanum onites, diluted with distilled water in 100, 80, 60, 40, 20, 10 and 5% rates was applied to soil and its effects on P. deliense population density in soil and severity of damping-off disease on bean plants caused by the pathogen were determined. As a result, all solutions containing oregano water over 10% rate significantly decreased pathogen populations in the soil samples taken every three days after the applications, and the least pathogen density was obtained with undiluted oregano water. Similarly, undiluted oregano water increased emergence rates of the bean seedlings and significantly decreased root and hypocotyl rot severity by suppressing the effect of the pathogen. In addition, application of oregano water supported the shoot and root development of the plants by decreasing the negative effect of the pathogen on plant growth. This study showed that oregano water was an effective alternative against Pythium related damping-off disease.