By the time Iowa attained statehood, its Territorial Legislature had created 44 counties. On 15 January 1851 the new State Legislature created another 49 counties, defining them by lines of survey. O'Brien was among the 49 units thus created. It consists of four survey townships, each formed by 36 square miles arranged in a 6x6 layout, for a nominal 576 square miles total area. The county was named for William Smith O'Brien, a leader for Irish independence in 1848. The new county's first settler arrived in 1856; Hannibal H. Waterman brought his wife and a daughter to the southeast portion. His homestead formed the nucleus of the county's first community, and a small building was erected there to function as a courthouse. By 1872 the county comprised several small communities, and residents of those groups felt the county's business should be conducted in a more central location. An election was held, the outcome of which was to create a new county seat at its midpoint. A 40-acre tract at the center of the county was surveyed and platted; its name was created from the initials of eight persons involved with the platting. The previous courthouse building was transported to this new location. In 1874 a 35-foot square building was erected on the present location at a cost of $2,000, to replace the first structure. It was replaced in 1887 by a larger structure. The County Board of Supervisors authorized its construction, at a cost not to exceed $5,000. To keep within this limit, members of the new community agreed to haul the building materials without charge from railroad stops at Sanbor and Paullina. The present courthouse was completed in 1917, and is listed on the National Register of Historic Places.
Geography
According to the United States Census Bureau, the county has a total area of, of which is land and is water.
As of the 2010 United States Census, there were 14,398 people, 6,069 households, and 3,927 families in the county. The population density was. There were 6,649 housing units at an average density of 11 per square mile. The racial makeup of the county was 96.0% White, 0.5% Black or African American, 0.1% American Indian, 0.6% Asian, 2.0% from other races, and 0.8% from two or more races. The county has a 3.8% Hispanic or Latino background. There were 6,069 households out of which 26.0% had children under the age of 18 living with them, 55.1% were married couplesliving together, 6.1% had a female householder with no husband present, 3.5% had a male householder with no wife present, and 35.3% were non-families. 31.5% of all households were made up of individuals living alone and 33.1% had someone living alone who was 65 years of age or older. The average household size was 2.31 and the average family size was 2.89. The county population contained 25.7% under the age of 20, 4.6% from 20 to 24, 21.3% from 25 to 44, 28.1% from 45 to 64, and 20.4% who were 65 years of age or older. The median age was 43.6 years. For every 100 females there were 95.80 males. For every 100 females, there were 99.1 males. The median income for a household in the county was $44,018, and the median income for a family was $58,127. The per capita income for the county was $24,771. About 6.3% of families and 11.0% of the population were below the poverty line, including 15.7% of those under age 18 and 8.3% of those age 65 or over.
