Summary Multiphase separators are essential to chemical units, especially in the oil and gas industry. Because separators are usually the first equipment installed in the oil and gas units, their performance directly affects other equipment. A two-phase oil-water separator was investigated in this study. Based on the average residence time reported for the pilot separator, the computational fluid dynamics (CFD) simulation was validated. The multiphase volume of fluid (VOF) theory in Ansys Fluent software is used for the simulation. Initially, the effect of halving the geometry from its line of symmetry was investigated to see if the halved geometry could replace the complete geometry in the simulation. After validation, the effect of four different inlet deflectors on the mean residence time (MRT) was investigated. A spherical inlet deflector and three plate inlet deflectors with varying angles were tested to examine the effects of inlet deflectors. Among the plate inlet deflectors, the one with an angle between the plates of 105 degrees had the highest MRT and the separator with an angle between the plates of 135 degrees had the lowest MRT. This is because of the fluid conduction by the separator and its collision with its wall. The spherical input deflector increases the MRT by 8.9% compared to the separator without the input deflector.