O’Brien County is uniform in shape, a perfect square, 24 miles each way. 16 townships of thirty-six square miles. Three rivers, the Floyd, Little Sioux and Ocheydan. Smaller streams are Mill Creek and the Waterman Creek.   The County was named by the Iowa Legislature.  It is named for an Irish Patriot, William Smith O’Brien.

O’Brien County was without a settler until the spring of 1856 when Hannibal H. Waterman, his wife Hannah H. Waterman and their child, Emily landed in O’Brien County with two yoke of oxen and a small amount of household goods. Mr. Waterman exercised his right as a squatter by filing a claim at the government land office in Sioux City and settled on what became known as The Waterman Place, the northeast one-fourth of Section 26 of what is now Waterman Township. It is an area five miles northwest of Peterson. The next year the first white child born in O’Brien County was the Waterman’s daughter, Anna, born May 30, 1857.

In 1872 an election was held to determine if a permanent location for the courthouse should be set up. To get the courthouse out of the hands of the gangs, it was decided that it should be moved to a 40 acre tract in the exact center of the county which later became Primghar. The courthouse from Waterman Township was then moved to Primghar on the north side of the square.

In 1874, a courthouse was built on the present location at a cost of $2,000. This building was about 35 feet square.

In 1887, the board decided to build a new courthouse for a total cost of $5,000 which was the highest amount the board was allowed to appropriate at that time. The actual cost was $6,000, so the good people of Primghar signed written agreements to haul the material without cost to the county from the railroad stations at Sanborn or Paullina.

On November 3, 1914 a special election was held to vote on building a new courthouse. The cost was $160,000 and it was completed and occupied in April 1917. This is the same court-house that is in in use today.