O’Brien County is located in northwestern Iowa in the second tier of counties east of the South Dakota state line and in the second tier south of the Minnesota state line. It lies partly in the "Wisconsin drift soil area and partly in the Missouri loess area. The soils of the county are mainly of loessial origin but a considerable area, particularly in the eastern part of the county, is of glacial origin.
Stevenson, W. H. and Brown, P. E., "Soil Survey of Iowa, Report No. 43—O'Brien County Soils" (1926). Soil Survey Reports. 34.