The first white settlers arrived in 1835 led by Nathan Boone, youngest son of Daniel Boone who acting on instructions from Stephen W. Kearny selected it to be the first site of Fort Des Moines on a high ridge between what Skunk River and Des Moines River. The ridge was originally called the Narrows.
The town was formally platted in 1844 when William Canfield who had a trading post on the Des Moines River moved it to Oskaloosa. It became the county seat in the same year.
In the 1880s more than a million tons of bituminous coal was mined in the area.
According to the United States Census Bureau, the city has a total area of 6.9 square miles (17.8 km²), of which, 6.9 square miles (17.8 km²) of it is land and 0.04 square miles (0.1 km²) of it (0.29%) is water.
There were 4,603 households out of which 29.4% had children under the age of 18 living with them, 49.2% were married couples living together, 10.1% had a female householder with no husband present, and 37.8% were non-families. 32.8% of all households were made up of individuals and 15.7% had someone living alone who was 65 years of age or older. The average household size was 2.28 and the average family size was 2.89.
Population spread: 24.1% under the age of 18, 11.2% from 18 to 24, 26.2% from 25 to 44, 19.9% from 45 to 64, and 18.5% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 94.9 males. For every 100 females age 18 and over, there were 91.3 males.
The median income for a household in the city was $34,490, and the median income for a family was $42,138. Males had a median income of $33,830 versus $23,698 for females. The per capita income for the city was $18,721. About 10.6% of families and 13.7% of the population were below the poverty line, including 18.0% of those under age 18 and 11.6% of those age 65 or over.
In the city's town square is a bronze statue of Chief Mahaska, the 19th-century leader of a Native American tribe called the Ioways, for whom Mahaska County was named. Recently restored, the artwork was completed in 1907 by an Iowa-born sculptor named Sherry Edmundson Fry (1879-1966). At the time it was commissioned, Fry was living in Paris. He returned to Iowa the following summer to make preparatory drawings of Mesquakie Indians at the nearby settlement at Tama, Iowa, and to collect Indian artifacts and other reference materials. Returning to Paris, he began on a clay scale model, which he first showed at the Paris Salon in 1907. A year later, he exhibited the final full-sized sculpture, for which he won the Prix de Rome. Soon after, it was shipped to the US, and arrived in Oskaloosa by railroad in September. The formal dedication of the statue, which was attended by a crowd of about 12,000 people, was held on May 12, 1909.
Also in Oskaloosa are two private homes designed in 1948-51 by American architect Frank Lloyd Wright. Typical of his Usonian homes, these include the Alsop House at 1907 A Street, and the Lamberson House at 511 North Park Avenue.
There are also many geocaches in Oskaloosa. See geocaching.
The Southern Iowa Fair is one of the largest traditional county fairs in Iowa. Midway rides, 4-H Fair booths, livestock exhibits, and a petting zoo are just a few of the aspects of this traditional fair. Campgrounds are also available to those wanting to stay at the fair site for days.
Art on the Square is an annual event held in the downtown on the city square. This event features local artists as well as artists from various locales.
The Sweet Corn Serenade is another annual event held in the downtown on the city square. This even has musicians playing on the bandstand. It also features games, but the main attraction is the corn on the cob that you can get, along with many other foods.
Yet another annual event is the lighted Christmas parade, which usually takes place in early December. This parade features the Oskaloosa Marching Indians playing "Jingle Bells"