To efficiently separate water containing components originating from coal tar or crude oil and forming an o/w emulsion, such as ammonia-containing wastewater from a coke oven of an ironworks or wastewater from an atmospheric distillation unit, an FCC unit, an RFCC unit, or a desalter of an oil refinery, into oil and water. A method for separating an o/w emulsion into oil and water includes adding a phenolic resin to an o/w emulsion including a component originating from coal tar and/or crude oil. A coagulant is preferably further added to the o/w emulsion. An organic coagulant is particularly preferable. It is possible to efficiently separate the o/w emulsion into oil and water in a short time to produce treated water in which the amount of oil components has been reduced. Since a phenolic resin is not an inorganic substance, reusing separated oil components including the phenolic resin does not impose any problem.