There are 7 US Post Offices in Mcclain County serving a total of 38,815 residents equating to 5,545 residents per post office. It's estimated that approximately 61,953 packages are handled across the 7 county post offices annually. McClain County is a county located in south central Oklahoma. As of the 2010 census, the population was 34,506. Its county seat is Purcell. The county was named for Charles M. McClain, an Oklahoma constitutional convention attendee. McClain County is part of the Oklahoma City, OK Metropolitan Statistical Area.