A method of preparing a positive electrode active material includes mixing a lithium raw material with a high nickel-containing transition metal hydroxide containing nickel in an amount of 60 mol % or more based on a total number of moles of the transition metal hydroxide and sintering the mixture to prepare a positive electrode active material, wherein the sintering includes a sintering step of heat-treating at 700° C. to 900° C. for 8 hours to 12 hours, a cooling step of cooling to room temperature, and an aging step of having a holding time when a temperature reaches a specific point during the cooling step. A positive electrode active material which is prepared by the method and has a reduced moisture content, and a positive electrode for a lithium secondary battery and a lithium secondary battery which include the positive electrode active material are also provided.