A system for crude oil desalting and dehydration includes a separation vessel defining a cavity extending along a central axis having a substantially-vertical orientation. The separation vessel comprises a first distributor configured to distribute a mixture within the cavity of the separation vessel, and a second distributor configured to distribute a wash fluid within the cavity of the separation vessel. The mixture comprises crude oil, water, dissolved salts, free salts, or combinations thereof, and the wash fluid comprises an aqueous fluid. The first distributor is disposed within the cavity below the second distributor, and an interface level controller is configured to detect an interface between an oil phase and an aqueous phase and to maintain the interface within a predetermined range within the cavity.