On a simple east/west basis, Western Oklahoma is popularly considered that part of the state west of I-35.
[1][2] I-35 creates a north/south line through the approximate center of the main body of the state (i.e., without regard for the Oklahoma Panhandle), passing through Oklahoma City, the state capital.
[4] The 14 counties of Southwest Oklahoma, called Great Plains Country, do all fit west of I-35, including the easternmost counties in the grouping, Stephens and Jefferson.
[4][5][6] But, while most of the 16 counties of Northwest Oklahoma, called Red Carpet Country, are also west of I-35, the two easternmost in that grouping, Kay and Noble, each have some land area east of I-35.
[9] And, the department has established a South Central grouping of 7 counties, called Chickasaw Country, some of which, like Carter County, have more land west of I-35 than east.