The most populated county in the state of Oklahoma is Oklahoma County, home to Oklahoma City. With a population of 792,582 residents, Oklahoma County has experienced a 9.96% rate of growth since the last census in 2010. Coming up after Oklahoma City in terms of size are Tulsa County (648,360) home to the city of Tulsa, Cleveland County (281,669), Canadian County (114,447), and Comanche County (120,422). These are the only Oklahoma counties with more than 100,000 residents. All experienced population growth since 2010 except Comanche County which declined 3.97%. The most significant growth was in Canadian County with an impressive growth rate of 24.16%.
The least populated county in Oklahoma is Cimarron County. As of 2019, Cimarron County has a population of 2,153, having declined 12.80% since the 2010 census. Harmon County (2,664), Roger Mills County (3,656), Harper County (3,797), Ellis County (3,952), Grant County (4,326), and Dewey County (4,894) are other small counties in Oklahoma with a population of fewer than 5,000 residents. Of these counties, Cimarron County experienced the most dramatic population decline and Harper County experienced the largest growth rate of 2.70%.
Since the last U.S. census was taken in 2010, the fastest-growing county in Oklahoma has been Canadian County. With a rate of growth of 24.16%, the population of Canadian County now stands at 144,447. Behind Canadian County are McClain County (15.11%), Logan Country (12.45%), and Bryan County (10.82%). The county with the biggest decrease in population is Cimarron County, which, since 2010, has lost 12.8% of its residents.