Pollution of water and soil environment with oil substances results from the common use of oil and its derivatives, as a basic raw material for energy production, in the chemical industry, in industrial plants and in transportation vehicles. Oil-water separators are devices used to separate petroleum substances (such as oils, fuels, lubricants, etc.) from rainwater or industrial wastewater. Their design is crucial for environmental protection, as they allow the effective removal of hazardous substances from water that can enter the environment, causing groundwater and surface water pollution. No information on a consistent, computational method for designing separators was found in the literature reviewed. PN-EN 858-1 only provides assumptions for the selection of these devices. The design of oil-water separators requires the analysis of many factors, including consideration of relevant standards and legislation to ensure efficiency and compliance with environmental requirements. This paper presents a new path forward in the design of oil-water separators, which could be an extension of PN-EN 858-1 to include the formulas used in the hydraulic calculations of these devices, and allow the sizing of separators to be optimized according to the size of the available land for their installation, while maintaining the required separation efficiency.