[3] By the time Iowa attained statehood (December 28, 1848), its Territorial Legislature had created 44 counties.

On January 15, 1851, the new State Legislature created another 49 counties, defining them by lines of survey.

The county was named for William Smith O'Brien, a leader for Irish independence in 1848.

His homestead formed the nucleus of the county's first community, and a small building was erected there to function as a courthouse.

[5] In 1874 a 35-foot square building was erected on the present location at a cost of $2,000, to replace the first structure.

To keep within this limit, members of the new community agreed to haul the building materials without charge from railroad stops at Sanborn and Paullina.

The present courthouse was completed in 1917,[6] and is listed on the National Register of Historic Places.

It has backed the Republican nominee in all but five elections in its history; its inaugural election in 1860, a tie in 1864, backing former Republican turned Progressive Theodore Roosevelt in 1912, and supporting Franklin D. Roosevelt during his two nationwide landslides in 1932 and 1936.