Newcastle is a city in McClain County, Oklahoma, United States, and part of the Oklahoma City Metropolitan Area. The population was 10,984 …Newcastle is a city in McClain County, Oklahoma, United States, and part of the Oklahoma City Metropolitan Area. The population was 10,984 at the 2020 census. Newcastle became a dot on the map with the opening of a new post office March 26, 1894 and a population of 25. The area had previously been served by the nearby William P. Leeper Post Office, opened in 1888, but closed in 1892 after Leeper was shot in a range fencing dispute. The mail office was established on the Minco-to-Norman road in Section 11, Township 9 North, Range 4 West. Eulalie V. Kelley was the first postmaster. In 1905, postmaster Alonzo Haun moved the facility to his general store in Section 14, Township 9 North, Range 4 West.